can't ride my boyfriend..

So, my boyfriend and I have been having sex for about a year now and everytime I try to ride him, it just slips out. I think it's because his dick is too small.  We have amazing sex in missionary and doggie style, but i just can't ride him. I'm not like some pro at it, but I have done it many times before and never had a problem until him. Is this the reason why? Also he is a bigger guy so could that contribute?