More often than not it's the visual, the idea and excitement of getting fucked by a really big dick. Can't deny that it turns me on too. But also often it turns out that a. he doesn't know how to use it, b. can't get it hard or c. his ego is even bigger and having a big dick makes him think that's all what's needed. Don't get me wrong, I had great times with guys with a bigger than average dick but I also know the most satisfying times for me as a bottom are/were with guys who are average and knew what to do with it.
As long as it's big and thick enough to rub my prostate, or, if he knows how to keep going for a while (or more than once) and doesn't shoot in 5 minutes I'm a happy bottom ;-)
Doesn't mean I don't like to look at guys with a massive dick, it turns me on and I fantasize about it, but I also know it's certainly not a guarantee for a good fuck and I certainly don't reject guys based on their dick size. Unless - as @ErosWired says- they are really undersized. Simply because I think I won't get any pleasure out of it myself and it's just him having all the fun (I'm selfish). That goes for a 1-on-1 date. In a group, darkroom or bathhouse I couldn't care less, in those cases a dick is a dick.....