It usually is though. Cooking times however don't tend to scale predictably (except for microwaves).

Not sure I agree. In my very limited experience, I have found that cooking times for microwaves do not scale predictably. For example warming up 2 packs of something seems to use less than double the amount of time for 1. I have had issues in the past of doubling or tripling the cooking time for double/triple the quantity and having food overcooked.

This makes sense. When you see a microwave rated as 750 W, that's assuming there is a sufficient load inside the oven that can absorb all that RF energy. Usually there's not, maybe your food can just absorb 400 W, and when you double the amount of food maybe it can absorb 600 W.

Mainly this is because the outer ~3 cm of food is what absorbs RF energy, heating of the rest of the food is through thermal conduction.