Think of it this way. Assuming by "overweight," you mean that the extra 10 KG is fat, not muscle. So the two people will have the same amount of muscle fact, and ceteris parabus, that is, assuming max VO2, lactic acid tolerance, and fast twitch/slow twitch muscle fiber ratios are about the same, which would happen if the two are close enough genetically and in diet and training, should be able to express about equal amounts of power. The ratio you end up with is that the time of the heavier guy divided by the time of the lighter guy is (x+100)/x Newtons.
To give a more concrete example, I weigh about 87 kgs and can run a 5K in right around 18 minutes. If I dropped 10 kgs without losing any muscle mass and maintained about the same level of muscular and aerobic endurance, I'd cut my time to about 15:55. Notably, I was about 10 kgs lighter in high school and ran in the mid 16 minute range. I'd reckon I actually have far less endurance now than I did then, but I have much more muscle mass, and thus can express far greater amounts of power.