Well, let's see...
I've compared, casually, in the locker room before, but not side by side actual comparison. I mean, a guy will sort of eye me and I'll do the same, or some guy says (like one did just the other day after watching me in the steam room, then the shower, the drying off), "It's a shame to put that away" as I'm pulling up my boxers.
But real comparisons for me were done at sleepovers when I was in middle school or high school. We'd get to talking about sex, get hard (which at that age took practically nothing to accomplish) then we'd get the idea to compare. I typically came out on top of those comparisons, which may be why I instituted others. LOL
In college I remember my roommate and I comparing both soft and hard. It started with a "who's stronger....who's chest is bigger" type conversation and led to a natural conclusion. We'd already seen each other naked, so it was just another level in the competition, I guess. He used to get pissed off at me when I'd make better grades on a test we both took, so we were pretty competitive on lots of things.
Other than those situations where I was already in close quarters with the other guy or guys, I've only compared sizes side-by-side in situations where the other guy and I were already naked, already hard, and already close enough to compare. Such comparisons usually ended with some sort of reward for the winner that involved orgasms, so I'm not sure that fits in the parameters of the types of comparisons you're wanting to make. hehehehe