I think guys generally get submissive to guys with larger dicks because of instinct more then anything. They realize they are "inferior" in some primal way and want to do their part to help humankind evolve by letting a man with a bigger, superior penis impregnate his wife and then raise the man's children.
Of course it doesn't really play out like this, but that's where the instincts and feelings come from.
Bullshit. There is no altruism in evolution.
I am not submissive to large cocks, I am submissive to a dominant woman. A Domme that I respect and desire to please can have physical experiences with a big cock that my smaller dick cannot provide for her. When she lets a big cock fuck her, it is just that; a big cock. In that respect, it's no different than when I use one of her big toys on her: a man fucking her with a big cock is a big cock and nothing more.
She gets to enjoy larger men on occasion because--as a submissive--my goal is HER pleasure; I want her to enjoy herself. Of course, I love to watch/hear/help her get fucked by a larger cock so seeing it happen or hearing about it later gets me off too.
Submissive in all aspects of life? Hardly. Just because I have a small(ish) cock doesn't mean that I don't have an A-type personality. I am quite the normal man in that I have a competitive drive like a sumbitch, am not afraid of speaking my mind and standing up for myself, and make the first move when dating women. I won't say I'm cocky, but I am
definitely not submissive in all aspects of my life. Just because I submit in the bedroom doesn't mean that I'm a pussy in the rest of my life. If you ever met me on the street, you wouldn't think that my sole goal is submitting to a sexually aware, confident, dominant woman. You'd be surprised how difficult they are to find.