Yeah, I think for the 100% straight guys, it's all about curiosity, seeing how you compare in the spectrum. As straight guys, we don't get many chances in real life to compare.
For me, it's always been such a turn on seeing girls react to a big dick, hearing their thoughts on it, if it impresses them, if it turns them on. I can't explain why that turns me on, maybe because I used to be much smaller and fantasized about what it would feel like to have a big, confident dick to pull out and turn all the girls on. So, especially when I had a smaller penis, whenever I see a photo of a big cock, or a hung guy in real life, there's a fantasy aspect to it, like "If my dick was that big..."
So, that's what turns me on about big cocks, but I'm only into pictures or vids of big dicks, if there's a woman involved on screen, hearing and imagining her perception of the big penis.