Some thoughts on incorporating mass into GURPS 4e:

GURPS 4e uses Strength and Hit Points as imperfect proxies for mass. This works for human sized characters but leads to some odd results - falls are equally dangerous for creatures large and small and halfling are more effective at slams than seems logical. My proposed solution: creatures get a Mass Rating (MR for short) equal to the square root of the creature's mass in pounds rounded down. A typical adult male human would have a MR of 12, with the typical woman having an MR of 11 or so.

 (Brief digression: Yes, I know pounds are a unit of weight not mass, but GURPS works with pounds. Just think of it as "pounds the creature would weight under earth normal gravity.")

The simple version:

MR replaces HP for calculating falling damage and calculating slam damage. The net effect is that large creatures will be more effective at slams but more vulnerable to falls. Reverse the effects for small creatures. On the whole this breaks out even - I'd treat MR as just a feature. MR defaults to HP but it costs nothing to assign a different MR. I believe this is an option in GURPS Powers.

 Complex version:

In addition to the above, mass has other effects in combat.

 Takedowns: The creature with a higher MR gets a bonus to the Takedown roll equal to his MR minus the opponents MR. Apply this before fixing the contest of ST. For example using the GURPS Basic sample characters Sora (130 lbs, MR 11) and Dai Blackthorn (100 lbs, MR 10) Sora would get a +1 bonus.

Sacrifice Takedown: Add MR/3 (rounded down) if the attacker attempts a sarifice takedown - dropping and trying to pull your opponent down with you. Again apply before ficing contest of ST.

Pinning: The pinning character gets a bonus equal to his MR minus the opponent's ST. If the opponent's ST equals or exceeds the pinner's MR the pinner's weight is simply not enough to hinder the opponent.

 GURPS Gulliver suggests a number of places where mass can add to damage. Simply replace "square root of weight" with MR. A couple of examples:

Jump Kick: Damage bonus is MR/5 rather than +2.

AoA (increased damage): Bonus is MR/5 instead of +2. This could get big for large creatures but they get +1 per die under the standard rules.

This is arguably more realistic than standard GURPS but does give significant bonuses to more massive characters. Since MR defaults to HP, I suggest charging for MR differing from base HP at 1 point for every point of difference. So a character with ST 14, HP 14 and MR 15 would pay 1 point extra beyond ST for the added mass. Alternately, be sure to play up the limitations of extra mass and possibly require massive characters to buy Disadvantages reflecting natural encumbrance. Note that the difference between MR and HP isn't likely to be large for human sized creatures. For an elephant it could be over 60 or so - the elephant listed in GURPS Campaigns has a ST 45 and would have an MR of 109.

- DW