I have never found being on top easy if he's too big. It's more like a knees up position but you are just on top. It's no easier than say laying on your back with your knees pulled up. I also find it very hard on my knee joints to try to control the depth while on top. It's really a big pain as far as girth goes. It's easier to lay completely on top of him (like reverse missionary, with her on top and him on bottom) when it comes to not tearing. I can also put my pelvis higher (like near his belly button) as to not allow him to push too deeply inside.
That's really interesting because I've found the opposite to be true! I could be because I have a tilted uterus, though, I don't know. I guess this just goes to show that every woman is going to be different!
If I'm on my back and I lift my knees up, the closer I lift them to my chest, the deeper he seems to go, and it can be very painful. He really likes this position a lot, but I rarely let him do it and it makes me nervous. Putting my legs out straight prevents him from going too deep.
I dislike being on top in the "cowgirl" which is the positing where a man lies down on the bed and the woman straddles him. It hurts my knees, my legs get tired easily from trying to balance on top and not sit all the way down on his penis, and in order to use my arms to help guide myself while I move I have to lean forward or backward, which makes moving up and down his cock much more awkward. It's not something I can sustain for very long and I've never orgasmed in that position.
The chair solves so many of those problems for me. No knee pain, the leg position makes it so that I can go for an incredibly long time before tiring, the ability to grip something sturdy and use my arms and shoulders and back give me so much more control that it just works for me. The closeness of our bellies also allows me to rub my mons pubis against him as I move up and down, which stimulates my clitoris (sorry to be so graphic, but how else to explain?)
His penis is girthy, but I have no difficulty with in for vaginal sex, just depth issues. If it's been a few weeks since we've had sex, because of a business trip, I am tighter, but not too uncomfortable to enjoy sex. We sometimes use lubricant to help with that.