My last relationship was with someone who has a very nice bubble butt and strong thighs like sdg475's girl's. A bit less hip width but cheeks just as bubbly if not a little more. When you thrust in and make contact, most of what's in the way will move from the force of your body. Same goes for her riding on top. Other positions, especially if she's flexible, you get full, hard penetration despite the cheeks.
The only position where I noticed her ass being in the way was with her laying flat on her front, legs together and me in plank position just above her for long thrusts. That way her cheeks were often flexed. It's not a good position for full penetration but unless the guy is quite short, she will still love it. It's a very good angle for your cock to press and rub against the g-spot area. For deeper penetration I would change my position to on my knees kneeling upright, straddling her ass. That way I could sit my weight on her or use the bed to move up and down for good penetration. I recommend this position, by the way, except guy stays still, weight on her and girl moves her hips around and up and down. It's a killer.
Overall I think the guy would have to be under 5" for those from-behind positions to be a problem.