Those kind of muscles were only going to be a hindrance to any fighting ability, so he had slim down in order to be able to move and strike faster and therefore with more power.
You must be a particularly scrawny goober. Your posts reek of resentment towards strong, powerful and large men.
Not to mention you're incredibly ignorant about muscle, speed and force.
In almost ANY non-sporting physical altercation, someone as massive, powerful and functional as Mariusz used to be would simply destroy an opponent. That he's lost so much is because he trains MMA, which eats calories and tissue like no tomorrow, and focusses on combat as sport, which by its nature requires excessive cardiovascular capacity. Timed rounds and pacing and severely limiting rules.
Real life fighting isn't reflected very intensely in MMA.