Water's basically incompressible, if some Michael Crichton book I read like 25 years ago is to be believed. My guess would be one or more of: the above; greate

Water's basically incompressible, if some Michael Crichton book I read like 25 years ago is to be believed. My guess would be one or more of: the above; greater heat capacity; or, something to do with surface area vs. volume meaning that at a large enough size water in a tire is far lower-pressure and easier to contain than air at a high enough pressure to keep it inflated, and/or, relatedly, something to do with heat dissipation from compressing gasses being really hard to deal with once you hit a certain surface-to-volume ratio. [EDIT] LOL, guess all these were wrong and it's just for the extra weight.