Your vagina, most womens vagina are not 7.5 inches long... they are much much shorter in lenght than that. A five inch penis would have no problem pressing on your cervix if thats the pressure you desire, in certain positions that shorten your vagina even more than normal. Doggystyle, Legs pulls up over your head should increase the depths your man can reach.
It sounds like this guy just had a good hit on your gspot, not to mention had you mentally arroused by the bug your friend put in your either that bigger = better orgasm. I am not discounting that it wasn't a pleasurable experience, sounds like you squirted and I'm sure it was amazing.
But I doubt its because of his bigger penis. I have a high sex drive, vaginal orgasms, clitoral orgasms and every combination there in. I've had penises that must have been 9inches and one that couldn't have been more than 3. My pleasure has always been more based on the spots they rub against (position plays the biggest role in this) and how much they have me turned on.
I don't think it coincidence that your friends been bragging about how much pleasure you get from a bigger penis, you then finding a bigger penis... and experiencing more pleasure. Sex is mental more than anything else. I think you you were met with a combination of mental arrousal and anticipation along with a man that knew how to position his body and stroke in a way that hit your spots the best.