When a fighter has a high amount of muscle with little body fat most likely that fighter has a higher level of fast twitch muscle fibers (type 2). What this means is that these fibers are made for more explosive less aerobic capacity. The easiest way I can put it is if you look at a sprinter who utlizes mostly type 2 fibers they are more muscular in nature. In contrast you look at a long distance runner with more type 1 fibers (high aerobic capacity) has very little muscle.
When a fighter like a Tyron Woodley or a Yoel Romero use alot of energy in a given fight sequence they will begin to tire. This is due to having that high type 2 fiber count which is a genetic hindrance to aerobic output. The good news is that this can be imporved through proper training and programming.
When I get a fighter that is mainly very muscular and have alot of power but their aerobic output is weak I start with their strengths. What this means is that I utilize the high power output in a sequence which can be for 5-10 seconds. Istead of letting the fighter fully recover after the high energy output I make them do whats called active recovery for around 10-20 seconds. This will progressively get harder once I see your recovery build stronger. In time the main goal is to get the fighter to be able to utilize their strengths while maximizing recovery between the output of power.
The point im trying to get across is to not be afraid of gaining muscle as a fighter if thats how you are genetically made then lets work with your strengths and improve your weaknesses. I most recently worked with a fighter by the name of Walt "Big Ticket" Harris a UFC Heavyweight with a simular problem. Walt is a very athletic and powerful fighter, and in the past he began to tire after huge outputs of energy. He was confused on why he would get this way even when he does long distance cardio and other conditioning that most fighter do to get in condition. I began to inform him that it really wasn't his training that was lacking rather his prescription was not on point. I told him that instead of trying to change your style we need to strengthen the recovery time between power output. When this is achieved higher outputs of energy along with sustained energy will be present.
Strength Conditioning Coach
FMS, ACE Certified
Sports Med, Exercise Science Degrees