system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 - When we gain weight, why is it hard to notice it but when you see a picture of yourself it’s super obvious?

I know I’ve gained weight but I don’t feel super fat yet but after seeing pictures of myself this weekend it’s obvious I need to jump on a treadmill...
gpt
Because weight gain is usually gradual. So you look in the mirror daily (or weekly, or whatever) and you look *about* the same you did as the last time you looked. You do it again, and you look *about* the same *again*. But if you compare the first time you looked to the 3rd, 4th, 5th, etc, you're seeing those gradual regular changes happening all of a sudden at once.  

Weight doesn't normally change much in a day or week even, but over many months those minor changes snowball into more major noticeable changes that's very clear if you compare pictures from different times.