You're assuming that the interior of the microwave behaves like an uniform field, so if you occupy 50 % of the volume, only 50 % of the power can be coupled to

You're assuming that the interior of the microwave behaves like an uniform field, so if you occupy 50 % of the volume, only 50 % of the power can be coupled to that object. This is not the case. There are some (big) losses in how power is coupled to the object in the microwave, but it's not 98 %. Also note that the power is coupled through the surface, so for power transfer it's mostly important that you have (1) big surface (2) surface depth exceeding penetration depth. The exact coupling is "dirty" (near field), so it doesn't just depend on surface area, but also surface structure, precise alignment etc. E.g. a ball of mashed potatoes takes longer to heat up than a ring/donut-shaped arrangement of the same mashed potatoes.