[Varyon]

A bit of necromancy, but I've recently been thinking about this and think I have an acceptable solution, at least for my purposes. Going off of GURPS standard of a meal being 1000-1200 kcal (thus 3000-3600 kcal/day) and assuming an average GURPS character to be a 25-year-old male who is 5'8" and 145 lb (the dead center of Average Build for ST 10), I'm getting a Resting Metabolic Rate of roughly 50% of consumption. The GURPS standard appears to be roughly based on active military (a full day's worth of MRE's provides around 3600 kcal), who from what I understand are typically at around Medium Encumbrance. The dead center of Medium Encumbrance for ST 10 is 50 lb, and an average character has Move 5 (which gets adjusted down to 3, but as we're going to be having everything adjusted equally, that doesn't matter here). This is important because the majority of the energy above RMR a character would expend is likely going to be spent simply moving around. So, 50% of energy is spent on RMR, while the remaining 50% is used on moving around 200 lb at Move 5, for a value of 1000.

For our pixie, said creature is around 0.15 scale (around 1 ft tall), which - going off of Kleiber's Law - implies an RMR of around 1/4 of a human. As weight follows the cube of scale, our average pixie would be around 0.5 lb. With ST 5, the dead center of its Medium Encumbrance* is 12.5 lb, for a total weight of 13 lb. Pixies have a Flight Move of 10**, for a value of 130. So, the pixie's RMR (which makes up half of consumption) is equal to 25% of a human's, while a pixie's activities (making up the other half) burn energy equal to 13% of a human's. This works out to a total consumption of around 19% of a human's, which I'm comfortable rounding up to 20%.

Now, I'm probably not going to require humans with ST above 10 (and Move above 5) to consume more, so pixies with high ST or Flight Move wouldn't either. If I were to do so, it wouldn't be terribly difficult to implement - keeping in mind the 25/13 split (or rounding this to a 25/15 split), a pixie with ST 15 (the maximum allowed for them) and Flight Move 15 would be shifting 113 lb (assuming the pixie still weighs around 0.5 lb), for a value of 1695; that boosts the "13" part of the split to "169.5," for 97.25% of human consumption (they'd eat pretty much exactly the same amount as a human). A comparable human, meanwhile, would be around ST 20 and Move 8, shifting around 350 lb (assuming still around 150 lb), for a value of 2800. This changes the human ratio from 100/100 to 100/280, for around 190% of normal human consumption.

*Using Medium Encumbrance may not be entirely appropriate here. While their food supplies would be expected to be proportionally heavier, and I envision my pixies using weaponry that is proportionally larger than what humans use, this doesn't seem like it would be enough to go from hauling around 1/3rd body weight in gear (a human with 50 lbs of stuff) to around 25x body weight in gear (a pixie with 12.5 lbs of stuff).

**Realistically, flying is likely less efficient than walking/running, but as GURPS doesn't treat flyers as requiring more food, neither will we.

NecroEDIT: Was thinking on and opted to revisit this recently. It occurs to me that, given roughly equal levels of activity, the "doing stuff" part of necessary food expenditure simply scales linearly with BL, and thus with the square of ST. Given ST scales linearly with height, this makes things easier - if the character's ST is equal to 5x(height in yards), they require 3*(ST^2)/100 standard (~1000 kcal) meals per day. If the character's ST differs from the above, necessary consumption is the average of 3*(ST^2)/100 and 3*((5*height)^2)/100. For an ST 5, 0.3 yard tall pixie, that's an average of 0.41 standard meals, or around 410 kcal, per day. For an ST 15, 0.3 yard tall pixie, it's instead around 3.41 standard meals, or 3410 kcal, per day. For an ST 20, 2 yard tall human, that's an average of 7.5 standard meals, or 7500 kcal, per day.