Theory: Chara is a fat fucking chud
So, as we have all noticed, Chara's sprite is a re-colour of Frisk's. So they have the same proportions. From the front, you can see that both Chara and Frisk's bellies are so wide and pie filled that it makes their arms look skinny because their fatass bellies are covering a part of their arms.
"But what if they just have skinny arms and they aren't actually fat!" I hear you cry. That's why we have exibit B: Frisk's side sprite.
If you notice, when you look at Frisk from the side, you can see that their belly is so fat that it extends past their feet and it is remarkably round (and full of pie).
Since we have established that Frisk and Chara have the same proportions, then it is reasonable to conclude that if Chara were to turn to the side, they would reveal a belly fat enough to rival Frisk's.
In conclusion: Chara is a fat fucking chud (as well as Frisk, but shh this ain't about them.)
I will not be hearing any counter arguments.