Tell him it hurts you. He should stop. I hope you are telling him this. He needs to know. You'd stop if you were hurting him, no? Do not be timid about this. Make him adjust or stop when it happens, every time. Sex is not supposed to hurt unless you want it to.
Stuff I've learned from my doctor, you should talk to yours:
If it hurts and you don't normally have a sensitive cervix (cervixes can be touchy, mine has become so over time), it's not good for you, but not terrifyingly bad. You just don't want scar tissue/tissue adhesions in your abdomen (this is unlikely but possible), they can cause pain themselves and only surgery corrects that. Don't be scared - this pain happens every now and then to everyone if they're with a big enough guy, or if they misjudge how warmed up they are for sex, but if it's a regular thing (and it never should be since you should tell him and he should stop every time it hurts), that's probably not good. And actually, bleeding is probably ok (this can happen to some without pain) as long as it's not a lot and you should get checked out if any at all is happening as well in case of STDs (especially HPV) or pelvic inflammatory disease (very bad news). It's also possible he's hitting your bladder, which if full or close to can hurt (you might piss blood/pus, not nice to look at but also not inherently bad - just make sure it doesn't happen a lot, same reasons as for cervix). Ask a doctor/Planned Parenthood (you can call them if you want to be anonymous, or the San Francisco Sex Info Hotline (email them or call:415-989-SFSI (7374)(US/Pacific time)3-9 PM Monday through Thursday,3-6 PM Friday). If you want him to fuck you hard you need to only do that in positions that are shallower, like Captain says, or you can try one of those giganto cock rings that are the size of your hand, worth a shot. Also make sure he takes a lot of time on foreplay. He should be used to this kind of thing if he's that hung, as this has likely happened with other ladies before, and if he hasn't had this happen then he especially needs to know. He'll probably be proud. Worried, concerned, super careful not to hurt you, sure, but also proud. It might also just be that he's too big to fuck anyone hard, too bad for him. It's also possible you are more sensitive cyclically - track your cycle to see when you might hurt more, if there is a pattern he can fuck you hard sometimes and sometimes not. And finally, get STD tested just in case. It's not likely the problem, but if he is a new partner, and something unusual is going on with your junk, just to be safe might be worth checking things out.