It’s probably a combination of things.
anatomy is one. All vulvas and vaginas are different. Heavier women will have more fat around the mons pubis that can make stimulation during PIV harder to obtain Vs lighter women as there’s more padding. Though the reverse could be true in other women as the padding could mean more friction/pressure.
outer parts of the clitoris are different sizes too. If it’s larger it’s likely to get more stimulation from PIV than if it’s smaller.
sex position helps too but this all depends on your own mobility.
there’s lots of health conditions that can impact it too. Women are more likely to suffer from lymphoedema, we can end up with birth injuries or difficulties just from pregnancy itself such as continence issues.
I’ve only PIV orgasmed with one partner. I was significantly lighter, younger and more mobile and more hadn’t had a baby yet so could get into the positions that worked. He was also girthier but average length which I believe helped me/us. But I did put more effort in with him. Because if I didn’t I wouldn’t get my orgasm.
ive been unfortunate that most of the men I’ve dated haven’t been interested at getting me off in any way because it takes longer and their disinterest in general makes it harder to be in the mood so the likely hood of PIV orgasm is ZERO