I'm gay but twice I've seen two of my str8 buds' erections and made a mental note.
1. A buddy and I both had bad sunburns after spending the day at the pool. Went out that night and had a lot to drink. Got back to his place and I decided to crash on the sofa. Both joked about sleeping naked because of our burns. Woke up in the middle of the night and saw him standing in front of the refrigerator with the door open. From the fridge light, I could tell he was quite naked and quite erect.
2. In college I knocked on a buddy's door one afternoon. He cracks the door just enough and it was obvious he had been sound asleep. Dark room, messed up hair, big sleepy face and wearing tightie whites... Apologized for waking him but also caught a glimpse of some major wood going on. He was either too tired to care or too tired to hide it well. Had seen him soft in the shower before and it was quite huge but this boner was pressed to the side in his tight briefs. I could see the ridge from his cock head which was stretching out past his hip.
So while we didn't really compare with each other in both cases, I certainly made mental notes and made my own comparisons.