system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come if fat is stored up excess energy, why can't heavier people do more athletic activities than skinnier people?
gpt
Two reasons:

1. fat is long-term storage, which means it is used to slowly release energy when you are low on food, not quickly release energy when you are doing exercise.
2. fat is heavy, moving heavy objects is hard

EDIT: heavy in the sense of: you carry around like 20+ kg on you if you are fat, not in the sense of "dense"