MisterK, your reply prompted a thought. The goal of the spell, overall, is to shrink or enlarge - it's a pretty classic fantasy idea, that a spell caster can do that to themself or something. Generally, as noted in the thread Elrich pointed to, the idea is about height, not mass. Few players sit and ponder mass changes, but growing taller or shorter is appealing in some situations. The no-ST bonus change rule in the spells as they stand is more of a play balance issue.
So I'm considering going with the spells changing height AND mass, and keeping height-width-depth proportions. What that means is a density change, so if someone shrinks to 10% height and mass, their density goes up 100x. Doubly size cuts density to 1/4. I can live with the idea that those density changes are sustained by the magic, and that they offset the size changes such that the ST bonus remains the same. A 12' tall, 1/4-density giant can still have a "meager" +10 or whatever they had at 6' tall. And that 7" tall miniature person is 100x as dense, so their "little" slap carries their normal ST bonus.
Of course, their 2" sword is not going to do the normal damage, it just can't cut that way!
I can see why you would do that, but strength is not merely a function of mass, far from it - even raw strength with a swing is a function of muscle power (to swing a given weapon mass) and of swing arc reach - this is why impact weapons tend to have most of their mass concentrated on the business end. Arc reach is a function of size, so, muscle power being equal, the one with the longest reach will not only have the advantage of reach but will also hit harder.
If it hits. This is why I stated that I increased the *raw ST* and not the *ST modifier* - raw strength is the ability to lift, carry and swing. ST modifier is the ability to swing effectively, and it requires coordination with that particular body size, muscle mass and weight repartition. A person shifting size would likely be suddenly very much off-balance, even more so if mass doesn't increase proportionately.
Changing density is a huge issue for me because of the side effects - if you're 10 times less dense (slightly more than x2 size increase on each axis), your average density suddenly becomes 1/10th that of water - your *blood* becomes 10 times less dense than it usually is. Conversely, if you shrink by a factor of 5, your density would increase by a factor of 25 (taking into account your rule of mass change) - or denser than *iridium* (and likely almost immune to most normal weapons). And because of the relation between size and momentum, you would have the same mass but 1/5th the momentum, and I would assume you would largely be unable to move.
Honestly, I think it's better to forget about most of the consequences : assume that flesh stays flesh and, as such, of similar density. Size changes alter mass in proportion in order to maintain density (which means that magic creates or removes mass temporarily, and quite a bit of it), and while raw strength is altered, effective strength is not, but the size change affects impact in a linear way (so a size increase of 10% will generate 10% more impact, or concussion hits). If the character goes into the large or above sizes, allow them to have monster-like benefits from it (I or II), increase pace, but reduce initiative and DB.
Another, potentially interesting way to handle those spells would be to consider them as self-regenerating all-senses physical illusions that also affect the target - the target feels their new body, other people feel the new body, but the new body does not *really* exist - no more than a physical illusion does, anyway. It solves all the strength and body mass issues, but I'm not sure it does not create others
One problem with that interpretation is that Illusionists would be jealous of such spells, given that replicating it with their own spells would require significantly more power :p
