When you gain weight, you also increase the muscles needed to carry that weight around. If you see someone obese at the gym doing the leg press, you may be astonished at how strong their legs are. When you lose weight, you don't need that muscle anymore.

Anyone can put up impressive #s on a leg pres. Try the bench press instead. No one impressed by leg press.

In regard to the oft claim of obese people being stronger or more muscular, not really. Studies show that obese people carry only a tiny extra 'lean body mass' compared to non-obese people when matched for height, age, and gender, and much of this extra mass is organs, not muscle. Otherwise, the extra weight is just water. Sometimes it is even less because obesity impairs movement, leading to muscle loss due to inactivity.

If obese people seem strong it is because the fat reduces the range of movement for certain lifts like the squat and bench press, so it's possible for obese people to put up impressive numbers owing to having to move the weight less distance. Same for pushing movements, e.g. linemen, as being heavier means more kinetic energy, but this is not the same as being stronger in the sense of more muscle output. This is why obese people are not that impressive at arm curls or grip strength relative to weight, but wirey guys can curl a lot relative to weight or have a lot of grip strength. An obvious example of this is overweight women having worse grip strength compared to men; the extra fat does nothing.