Living tissue (including fat cells) need a constant supply of blood and oxygen. The bigger you are, there's just physically more flesh to keep supplied. Your heart has to push a bigger volume of blood through a longer length of tubes to reach all the parts.

It's like how if a city could only have one water-supply pump station, and could only use one diameter of pipe, as the city got bigger and added hundreds of houses further and further away, that pump would have to run more often and at higher pressure to keep up with the flow being drawn from everyone's taps.