The cock actually looked bigger in recent issues and a few characters (Sarah, Alice) have described it as a big cock, like almost every porn comic character, the cock may vary from issue to issue, I think Daniel is the only one who kept his size, pretty sure David was smaller than Liam, now they may be the same size.
Anyway, even if he's smaller, what really matters is that he can use it, maybe David is really a good lover, while Peter is more of the brute that if it were not for his size he would be lame at sex, Aaron's aggressive attitude, while justified in a way, may translated to sex and I don't know if Sarah will fully enjoy it, not to mention he's virgin, he will probably do really bad the first time.