Should i tell my boyfriend how big he is?

ok, i dont mean big as in down there. he is big like on the fat side. he's at least 150 pounds overweight. when we make out, i always have him get on top. now im pretty fit and on the athletic side, and maybe only 5 pounds overweight that im working on. but he likes it when i straddle him, BUT, i have a bad hip, and if it gets too far stretched, it pops out of it's socket and hurts really bad. to straddle him i have to seriously stretch my legs, and then when that happens, my knees keep me up, and i cant properly straddle him. and it is because he is so big. i really cannot do it, everytime i do, i can feel it about to pop out. we have never had sex yet, but if we do, ill just be scared that my hip will pop out. we are both adults. should i tell him why i cant/wont straddle him?