I'm not an expert but I love boxing and MMA.
I think that size (which is one thing) and muscularity (which is another) can both play a role. But fighting potential is influenced by many more factors too.
Technical mastery, speed of reacting, psychological mindset, neurological nature, pain tolerance, natural explosiveness, instincts, strength of the adrenaline system, etc. All play a role too.
So size and muscularity rarely make a fighter great by themselves.
I also think that size is more important than muscularity. By size I mean the natural size of the body structure: height, limb length, size of head, size of hands, etc.
Muscularity by itself doesn't do much. Muscle size, if it doesn't make you stronger and more powerful wont matter. So it's not the size that is important but rather what those muscles give you.
I would say that the only advantage that muscularity has by itself is psychological: a super muscular body can be intimidating to others, putting them in a negative mindset when they start the fight; and it can give you confidence in yourself, which will put you in a positive mindset.