Size - Mars and Mercury are both quite a bit smaller than Earth. So they lose heat faster for essentially the same reasons that your cells have to be small: sma

Size - Mars and Mercury are both quite a bit smaller than Earth. So they lose heat faster for essentially the same reasons that your cells have to be small: smaller spheres have a higher surface area to volume ratio! While in neither case is diffusion the only process at work, it's a critical limiting factor in both cases. Diffusion of heat, diffusion of nutrients: it's all the same math!