I may understand what you mean, that is, ufc players do not have as developed muscles as bodybuilders, and their fat content is extremely low. Some players even have big bellies, such as the Big Belly Buddha.

Why is this? Because it is unnecessary, mma events like ufc don't need such developed muscles. Overdeveloped muscles are not even conducive to fighting. For example, too large pectoral muscles are not conducive to the movement of arms, and appropriate fat can also play a better protective role. In a word, it is because different sports have different requirements for the body.