OK, if you're already doing her that for her, then I do see why you would be wanting her to reciprocate. I was imagining y'all were still just in the heavy makeout phase.

I think that's good advice to just ask, kindly and with as much humor as you can muster, "How about it's my turn now?" See what she says. She may be afraid of your size, again, may have some preconceived (or experience-based) notions that she won't like it, may be afraid you might expect her to swallow. It's worth exploring whatever's making her hesitate.