@Myra LOUIS he wasn't always broke. + Manhwa dynamics. + He's still working his muscles so maintaining what was already built means he won't lose bulk as easily.
The fact that his father owned the gym means he's been working out from childhood and probably bulked up over time.
While genetics determine *part* of someone's physical build you can look at any famous body builders pics from teenage years to adult body builder and you can see their frame change with the work they put in.