IMO it depends on a number of factors. Strength is obviously one, as well as overall size, but agility and range of motion are also very significant. Then there are intangibles, such as fighting spirit and overall skill level (or how deep your bag of dirty tricks is, maybe). And in all of these things, the degree of difference between the two combatants also comes into play. For instance, a 90-lb geisha girl might be quicker, but the odds are she's going to get destroyed by a 180-lb fitness queen... unless the geisha has a black belt and the fitness queen is a lummox.
So to answer your question, I'd say being more muscular is AN advantage but not the whole story.
-JM