I don't know how true that is. An old School friend of mine who was overweight as I was as a kid, appeared to have nothing to write home about in the locker rooms at School and believe me, I noticed the ones who did. When we were 19 we (8 of us in fact) went on holiday to Spain and by then he'd lost pretty much every ounce of the fat he was carrying and seemed very proud of his new body, specially his cock which seemed to be at least the size of my erection (7x6) while flacid. He wore nothing in the hotel rooms for two weeks...
Also, i've always been overweight and am bigger than average but maybe that's an activity thing, I've never been as inactive as my size suggests, specially during puberty.
Look back at my first paragraph. I was talking about how being active and healthy helps maximizes cock
growth potential. I think I might have sounded like having no fat is absolutely necessary for a huge cock in my last post though, but that isn't the case at all. Like all health issues moderation is key and the body is a very complex system and everyone is a unique case. Having fat is actually healthy and necessary, but just like water having way too much or none at all will literally kill you. It doesn't help though when everyone learns though the media that your either big and full of fat, or your small and slim with virtually no fat. There is no deny though that way too much fat as in obesity levels of fat will ruin your cock size just take a look at this Howard Stern clip:
Smallest dick competition.
As for your friend losing a lot of fat likely helped your friend grow a bigger flaccid penis through greater blood flow and more building material for his penis just as Body builders often fatten up before pumping up their muscles.
Its also likely that you and your friend were just big guys and never obese. Honestly with today's skinny celebrities on TV anyone with a slight bulge on their belly is labeled fat and obese these days.
I also want to mention that you did bring up a good point about a person's activity level. I forget to mention that Blood flow is always vital to cock size 100% of the time. Cardiovascular health is absolutely necessary for getting an erection and it is no coincidence that Viagra was originally designed to prevent heart disease. Small guys have a huge advantage in this department because hearts don't actually grow much so huge guys are left with hearts that are more or less the same size and working much harder then their smaller counterparts.