I asked a few of my bigger female friends in university this same question. They had "friend zoned" me (epic shame for me, being friend zoned by a big girl), so they had no problem telling me what they thought was the truth.
But I always did well with big girls in university, even though I truly wanted a slim girlfriend. Just my thing...
Anyways, they told me the reason the bigger girls liked muscular guys is because bigger girls are very aware that they're big. Yet, there's a sterotype that girls should be made to feel like girls by their men, regardless of size. That a man should be bigger, physically; that he should look dominant (something a skinny has a hard time with when he's next to a 180lb woman who outweighs him by 30lbs); that he should be able to protect her and carry her over the threshold, and all that nice romantic stuff.
Bigger girls know skinny guys won't fulfill that fantasy they have of their ideal man. So, they go after a bigger guy. Muscular guys think that always and only means muscular guys, but it doesn't. They also go after bigger chubbier guys (when you play football in university, you see this a lot with the O-line); they go after taller guys (again, buddies on the basketball team got hit on by bigger girls a lot more than smaller girls).
It's simple programming; women want to feel like their man can swoop them up in his arms and carry her off. Every girl wants a strong prince charming. But if you're a bigger girl, you'll just need a slightly bigger prince charming, is all.