why do overweight individuals sweat more than ordinary people?
Well there are some overall, simplified answers, but every individual is ..well.. individual.

First there is the ratio of volume to area. As interaction between two mediums normally is depended on their common/shared surface. If we state, that an overweight person A has more volume than a skinny person B of same height, A also has a larger surface as B, but not in the same scale the volume is increased.

The easiest way to describe is with spheres, let's assume bellies are kind of spherical ;-) . A sphere with diameter 2r=1d (hint: one dimension) has a surface of π4r²=πd² (hint: 2dimensions; 2nd power 2²=4; d²=(2r)²=4r²) and a volume of $$\pi\frac{4}{3}r^3=\pi\frac{8}{6}r^3=\pi\frac{1}{6}d^3$$ (hint: 3dimensions; 3rd power 2³=8).
So, if we manipulate r it's manipulated to the 2nd power for surface and to the 3rd power for volume.
So, if we double the radius, we have 4 times the surface, but 8 times the volume.

Humans are biochemical machines, we take an energyform we can exploit to fuel our processes keeping us alive. These processes also produce heat, which is kind of a nice sideeffect allowing us to operate in cold climate. But in hot climate we need to get rid of the heat.

As I stated before, interaction is surface based. Person A has a lot more heat producing volume than B, but the surface isn't that much bigger, so A's surface has a higher urgency to interact with the medium around than B's surface (so more sweat for example). Also more tissue produces more heat. But again disclaimer: different tissues have different activities.

Furthermore fatty tissue has an insulating effect, which reduce interaction even more. Other way round the heatloss of an overweight person isn't as dramatic as for a skinny person in sudden cold.

But these are simplified answers, there's much more to it. Like bloodpressure, thyroid, biological activity. Which can be influenced by so many more things.