Its just the original artist's error. Most of the bulk in a bolt action is the stock, the receiver and barrel are very lightweight and compact for the barrel length, partially why some still like carrying bolt actions over semi-automatics. I think the original artist for Kar98 was going for a kind of upward perspective but kind of failed.
Someone on reddit did a whole analysis of the true size of the girls if the guns were drawn to scale and it is pretty funny.