My largest erections come when I haven't "gotten off" in awhile. When I'm on vacation, or visiting family, I am almost scared--no joke--at how large my erections can get. It's definately a blood-flow thing. The extra half-inch or so that is obtained from being super horney and hard can make a visually larger penis. If I'm with a new partner who hasn't seen my penis yet, I'll usually go four or five days without masturbating just to get this effect. Maybe it's a little vain, but I love teasing that 9-inch mark with a new woman. Once I'm getting consistent sex, I'm back down to the 8.5. But I don't think she notices then. First impressions are everything. LOL
But yes, the size of every guy's erection does vary.