

I'm guessing his colleagues think calling him 'fat" is a major insult, if anything it says more about them and shows their immaturity. The character you it was directed to wasn't even overweight, let alone obese (not that that is even relevant). In another manga's comment section, someone referred to a disliked character as being a "fat hyung' despite him also not being overweight, I guess for some people it is just used as an insult, a bit like how people think age, and calling someone "old" is putting a person down.
This comic I guess is set in South Korea, which also has a very image obsessed culture where plastic surgery is more normalised and people get "fat shamed" on television so maybe adults calling one another fat and ugly like you would expect from 5 year olds is less noteworthy, idk.

I think it because he wears those baggy clothes at work, so it seems so, even if he is so thin. Also they are just being mean. ┗( T﹏T )┛
http://www.mangago.zone/read-manga/reminiscence_adonis/nbt/4972c0c36943c060/Vol1_Ch8/2/
Lol when i was 15 i had figure similar to her always got mistaken has older cause of it. ╮( ̄▽ ̄)╭
Same! I don't know where your from but wen I was 15 I look as old as my aunt. I was even the same size as my mom(▰˘◡˘▰)
Right ever time i went out with my mom they thought i was her sister lol i was always like omg dont tell me i look that old already hahaha
Not that much, no that I did I looks a bit older, is that my mom look much younger than her age (she always did like looking in the young side of her self)
everyone has different body types
my sister had a body like that when she was 13, some people's bodies mature at an early age. It's pretty common