>>81706587 (OP)
In this case your small pelvis could be an advantage. A large man has to awkwardly spread the ass cheeks and thighs to get his cock in, effectively wasting a couple of inches. You can turn your pelvis perpendicular to hers and do something akin to a scissoring position, with your cock pointed down between your legs. Lean so that your pelvic bone grinds against her clit. Ideally you're both waxed so that you can make a suction seal with her juices, so grinding your hips to move your cock in her also sucks her inner walls slightly. Also lube up her anus before you start because your balls will be rubbing up against it and it *will* clench so better safe than sorry. It's an incredibly unmasculine way to fuck a woman but it feels really good for both parties and it works well enough with a 3 incher because your pelvic bone is doing most of the heavy lifting in terms of her pleasure.
