Lowering hits? That seems absurd. In RMSS the player will naturally have his returns degrade, after so many ranks he will still be spending tons of points but not get back much of a return. Around 10 (5 if he is min maxing), he is maxed out he will be spending points to get fractions of a hitpoint. All but the most stubborn go to developing other skills. And at a certain point he can't gain anymore hitpoints.
In any skill. I would suggest the problem is not the system,but that one of your players is cheating or Most likely does not understand that after so many ranks his return dimishes. What is it, the first 10 ranks are times 3. The next 10 ranks are times 2. The next 10 ranks are times 1. Then the next 10 ranks are .5 and it just keeps diminishing. But most stop buying points at this point and start diversifying on other much more needed skills. This happens with the Category also. So at best with talent law the guy may seem to have tons of hitpoints, but by the time he hits 5 (unless he is using talents improperly or the Gm has been to liberal) his returns start getting very much under control.
On top of that for the adolecence and first rank development there a limit of like 10 ranks per level. Even if his skill is super cheap, the best training package, and body development occupational (he can not pass that limit). I don't think he can have that many ranks in Body Development. After that he is limited to the number of ranks he can purchase per level. Even if he was a warrior, with the talent to add an extra rank, he could at most develop 4 ranks, and if it were occupational (I don't think that is possible) he would get 12 ranks. Again my point is, he would be maxed out by lvl 5. Not really gaining extra hits and such after that.
Most games should go 10-20 levels. He would of stopped gaining any appreciable increase in hitpoints around lvl 5 (if min maxing), most max out at 10. So what's the problem. At worse he is super for 5 levels. After that he becomes Mr. Ordinary for his rank.
Again most Gm's thro tons of mobs on that type of guy. Eventually he gets critted enough so that even Fate points can't save him and he dies. He has to start leveling again, while the rest of the group moves on.
If the Gm requires skill checks for things like direction sense, and other non combat skills regularly. The group needs many people skilled in it (because it takes forever to get decent in the skills). So the group naturally has to spread out in their skill set. And will brow beat the min maxers to spread out so they can enjoy their characters more.
My gm solves this by making the min maxer get seperated and lost in the woods by himself. And have to rely on skills like direction sense,star gazing survival, hunting, firestarting, etc. After being lost like that you know your character needs alot more skills than body development to survive
The rules of RMSS keep all skills diminishing Body Development is a skill, you can not even devolop the Category in. Your race decides the points and the hit return you get. Some races seriously drop on this. Maybe the problem is the person is not aware of his proper racial Body Deveopment rules.
If you seriously adjusted the rules for body development it would only be fair to lower the hits for the beast. As they too can have ridiculous hitpoints. And then you would have to lower the damage done on hits. Some hits are 20, or even 43 on a litely armored foe, some have talents or do double damage or more...plus the crit. And you have a problem with the players having to many hits?
And besides this is RM , I don't care if you had 1,000 hitpoints (I don't think that is even possible), one crit can kill you. Most players need the hits because Gm's throw 10-20 mobs at you at a time. Even if they are low level trash mobs these guys all have the potential to take the player out at any time. The rules for ressurection have such a penalty that it is almost not even worth resurrecting your guy (if you can find and pay a cleric who is lvl 12). Taking hitpoints away just seems like a GM power trip to me. But each is entitled to their opinion.
Add to this the fact that you have exhaustion points, and the tanks have to hide behind their sheilds taking a beating while trying to regain your exhaustion points. Add in penalties for your hps lost, bleeding, no instant heals (unless they are 5 levels lower) and other factors. And you need those hitpoints.
And if your still having problems with hits, change the type of mobs you send at them. Undead steal consitution just from being near you. The player can actually have full hits and be drained of constitution and be a lvl 1 created undead. If the group has no cleric (not many want to play clerics in our group, the cost is just to high to develop a D&D style cleric). It takes 5-10 levels to be able to have the stats to wear plate. Plus you need the restricted transcend armor skills. Skill development is very costly. And poof they have no way to really heal their constitution in long drawn out fights against undead. The end result they end up serving some necromancer.
Or maybe deadly plants, my wife was GM once, she loved those crazy poisonous plants. Try living without the means to heal that kind of stuff. Especially if you have half the hitpoints. Or maybe curses, which even the lower level ones are absurdly powerful. Maybe create a lair of rust monsters (would take some work on the GM's part) and there goes the armor (he will need tons of hitpoints). What if that enemy of yours overcasted by 5 levels and hit say a lvl 5 group with a lvl 8 or 10 spell? And the fact that Evil list (usually only used by the GM are super powerful) and I think all in all even a min maxed player is in balance.
Then there are traps. Add in some traps and it doesn't matter how many hitpoints you have. Bleed affects, gases, poisons.
Again our GM once used a "magic" circle, I stepped in and a demon was there, the demon threatened us with death, but instead made us do stuff for him. It took us a while to realize the things were illogical and it was an illusionist. Add in the mentalist Hourodi (spelling) that uses seduction and magic to work peoples emotions. A bard with his mentalist spells. The ways around it are endless. Thus the reason they are in balance.
A well balanced approach from a GM instantly tears down the hitpoints issue. I don't perceive it to be one. And crits can and do kill players regularly, even with fate points. Our High fantasy GM also rolls extremely lucky, so we have seen many crazy deaths. Add to this our hardcore gm when playing loves to overcast his spells (usually killing himself and taking many with him). And death at least in our group is all to real...hitpoints have never been an issue. I can't imagine a Gm wanting more player death, but hey that from my groups point of view.
My group has a high fantasy type GM and a super hardcore (your so poor you wonder why you even keep adventuring, because begging seems more lucrative type GM). I'm not kidding we don't even get training packages because he always puts us in jail or some story to take everything away. We don't even get horses because eveytime you go in a cave, they escape, an animal eats them or someone steals them. And even though some characters seem min maxed for the first 5 levels, by the time the group reaches 5, the others start catching up. At 10 it's all balanced.
So in our group the hitpoints work fine. It's balanced with all the other mobs in the game. Magic really balances it out (mentalism, illusions, summoned elementals). But I guess that is just our group.
But then again there are many GM's out there who only feel a game is balanced (and fun, usually only for them) if they kill all their characters off before they reach 5th level.
All that being said, and that i'm obviously against lowering hitpoints...
If you really want to control the hitpoints easily. Just lower the amount of points they get for each rank of body development for the races. It will scale down everyone's hitpoints...you should probably including mobs too. And you very quickly have solved your problem...Thus the beauty of RMSS (probably all RM) anything is easily fixed. The game is that flexible