Yeah, this is a trend that I guess authors use to make it less iffy to draw more intimate scenes with characters that are minors. I learned not to mind it, but it's unrealistic for a 18yo to be this bulkes unless he spent all the time in the gym and was naturally predisposed to gaining muscle
I think it's also about him being a panther, but an agile feline wouldn't be this bulked. Had he been a lion it would've made more sense

Somehow, I really love Yoontae's visual concept. The sharp yet droopy eye, the large mouthline, might be the artstyle but it really feels peculiar