As a working document/set of justifications, the above post - particularly with needing to back-reference other posts - is nasty to read through. Here's something (a bit) cleaner and more complete.
In addition to normal traits, characters also have a Mass and Volume, both expressed in pounds. Mass is actually "Pounds the character would weigh at 1G," Volume is actually "The space an average human of the given weight would take up." These assume a default of 160 lb; in a campaign with a different default, feel free to adjust the below values (if the default is 320 lb, every +40 lb is equivalent to the +20 lb values below, for example). Most characters have each of Mass and Volume equal to 8/5*(HP^2) lb. For Mass and Volume based on ST, substitute ST for HP. For Mass and Volume based on SM, multiply 160 lb by the cube of the linear ratio (SM 0 is 2 yards; SM +1 is 3 yards, or 1.5 times this, so multiply 160 lb by 1.5^3; SM -1 is 1.5 yards, or around 0.7 times this, so multiply 160 lb by 0.7^3).
Mass affects Grappling (see
Weight and Grappling, TG8), Knockback, Slams, and Falls. For the latter three, use (Mass/1.6)^(1/2) to determine your effective HP for these effects. Point cost is equal to this effective HP, minus 10.
Volume affects the weight and cost of armor and clothing. Simply multiply the weight and cost of any armor or clothing item made for the character by (Volume/160)^(2/3). Point cost is [-5*((M*100)^(1/2)-10)]; follow normal GURPS rounding conventions. Volume is constrained by SM - a character's Volume must fall between the averages of 1 SM smaller and 1 SM larger.
Characters with a Mass and/or Volume that doesn't correspond to the average for their SM, ST, and/or HP can purchase certain Advantages and Disadvantages to represent this. See the list below for examples.
Athletics/Movement/Carrying Capacity: You're carrying around more or less weight than you "should" be, so you can move more or less easily. Characters with a higher Mass than their ST indicates start out somewhat encumbered - every +(1/10)*BL lb is worth [-1] up to +2*BL lb, after which point every +(3/10)*BL lb is worth [-1]. Characters with lower Mass start out somewhat unencumbered - every -(1/10)*BL lb is worth [1] down to -2*BL lb, after which point every -(3/10)*BL lb is worth [-1]. These values are added directly to any encumbrance the character has, and can result in negative encumbrance, with effects as outlined in the following table (weight is the multiple of BL the character is carrying, Mass would be the total weight of an ST 10 character+gear to reach that level):
Code:
Weight Mod Move Mass
-5 +4 x1.8 60 lb
-2 +3 x1.6 120 lb
-1 +2 x1.4 140 lb
0 +1 x1.2 160 lb
1 0 x1 180 lb
2 -1 x0.8 200 lb
3 -2 x0.6 220 lb
5 -3 x0.4 280 lb
10 -4 x0.2 360 lb
Blowthrough: Bullets and the like rip through more or less of your flesh before shooting out the other side, causing more or less injury than anticipated. A character with higher Volume than his HP indicate has a higher blowthrough cap - +20% justifies Vulnerability x1.5 (Piercing/Impaling, only for calculating Blowthrough cap) [-10], +40% justifies Vulnerability x2 [-20], and so forth. A character with lower Volume than his HP indicate has a lower blowthrough cap - -20% justifies IT:DR x0.7 [10], -30% justifies IT:DR x0.5 [20], and so forth. You can determine the multiplier/divisor by squaring the character's Volume/(HP Volume).
Toxins: You've got more or less tissue to disrupt, granting you more or less resistance to poisons and the like. A character with higher Mass than his SM indicates can have IT:DR (Poisons, Occasional) [10/level], and higher Mass than default (160 lb) justifies Resistance to Poisons [3/+2 HT]. One with lower Mass than his SM indicates can have Vulnerability (Poisons, Occasional) [-10/level], and lower Mass than default justifies Susceptibility to Poisons [-3/-2 HT]. You can determine the multiplier/divisor as the character's (SM Mass)/Mass. For Resistance/Susceptibility, every
+1 SSR from default (160 lb) to Mass justifies
+1 to HT.
Consumption: You've got more or less mass to support, so you need to consume more or less food and water. As variable levels of consumption at any size are justifiable, this is the least necessary modification. That said, characters of higher Mass than default (160 lb) can justify Increased Consumption, while those with lower Mass than default can justify Reduced Consumption. Amount of food/water consumption scales as (Mass/160)^(2/3).
For characters of SM greater or less than 0, simply use the above guidelines. In addition to the above traits, every +1 to SM gives IT:DR (Everything but Explosions/Area Effects) [25/level], at x0.7, x0.5, x0.3, x0.2, etc. Every -1 to SM gives Vulnerability (Everything but Explosions/Area Effects) [-25/level], at x1.5, x2, x3, x5, etc. These assume you scale wounding modifiers by SM; if you don't disregard these. There are other SM-related effects, such as Reach, ability to be seen/attacked, etc, but these work out to an overall Feature.