Women assume that men can penetrate their large asses and not lose erection. Well some can and most can't and that is where the women have to rethink their position in doggy style. The number one reason women like doggy style is because its easy. Some women like the fact that they can bend over and spread and not do much else. Some women believe that gobbling the dick is the way to satisfy the man during doggy position.
Guys need to gain back control of this position and tease the woman. If you are dealing with a large ass then I suggest placing pillows beneath her so she can feel all of you. Then make her spread her cheeks and then go deep inside with slow small strokes. After a few seconds pull out and rub in between the cheeks and if she wants more she needs to earn it. If she doesn't follow the rules and keeps trying to bury your dick then you need to smack her ass for punishment.
Remember when dealing with big pussy or ass, that size matters. Big pussies and ass were created for big dicks, If you lack in size then you need to go at it a different way. Some men feel intimidated by something so big but that's because they have lack of size or they think they wont feel anything. You can feel all inside a big pussy if you have the woman positioned right. Pillows are your friend when trying to accomplish such a big task. Encourage also the woman to do kegel exercises to tighten up.
If the pussy is too big but you still want her then I suggest anal sex, this way you will have a good time and if she refuses then let her know you will find somebody who will.
Tip: Dip in , dip out , wait few seconds, tease , dip in ...........small slow strokes