I'm really not sure how to help you with this issue. I do doggy style and have done it many times, but vaginal entry really does depend on the size & shape of the penis and the angles involved. If your issue is that your penis can't make far enough to gain entry, then you may need to modify the doggy style and have her lay face down on the end of the bed so that her knees are on the floor and her butt is jutting out. Have her then spread her butt cheeks to give you as clear a path as possible. You then kneel behind her and place your erection into her vaginal. Hopefully this will give you the 'clearance' that you need.
Not wanting to kid you on this, but losing some wieght really would be a better answer (something to think about).
Wendy