I tried out -25 for A's vs Large, and -50 for As (and -25 for Bs) for SL, and found the results were fairly weak. . .I eventually dropped it, I recall not being 100% happy with the results, but it's been a while so I don't 100% recall the problem with it.
Could it be because with such a penalty the vast majority of the crits falls under the lower results (01-05) which indicates fumbling and/or weapon breakage?
I personally use an house rule of sort mediated from some very old ruling found on a website back in say 1998.
Non-human but non-large creatures (i.e.: normal animals) take criticals on the Large Creatures table with a modifier of +10/degree of severity of the critical (starting with +0 for "As").
Large creatures take critical on the appropriate table, ignoring As crits and modifying the result by +5 per rank of the crit starting from Bs at +0.
Super Large take crits on the SL table ignoring all As and Bs and modifying the result of +2,5 (round up to 3) per rank of crit starting from +0 for Cs. So a D is +3 and an E is +5, and a F is +8 and a G is +10 and so on.
I also use old RM2 Arms Companion different tables for L and SL crits (Large Crushing, Super Large Puncture and so on).