I'm not sure all muscle woman are looking for a bigger man.
Certainly no woman who has built significant muscle would look at a guy who was plump, soft, puny or similiarly out of shape.
But I think the bigger bodybuilders and powerlifters have had time to get their heads around the fact not many men have more muscle than them and they would show interest in smaller (in width and thickness only, not height) guys, so long as they were athletic enough to be aesthetically pleasing, say with decent abs and at least some pecs and guns.
I reckon it's the middle tier of muscle women like advanced crossfit and fitness types who would insist their man is more muscular, because they're so into muscle and not short of potential candidates.