It is all normal. My first few times it happened a lot.l Him slipping out easily. I would say for the first few times, try lying on your back with your legs up. I am also a big girl or used to be. Haev your legs on his chest or arround his waist. Let him enter you and slide in an out. Tell him not to move to quickly at first. It gets both of you used to the situation. Beleive me when I say you will get the hang of it all. We all do.