Yup, this is something along the lines of what I was going to say.
I think large breasts and large penises can be compared to a very limited extent. Yeah, both are visually stimulating and probably fun to play with, but the penis is actually used for sex and the size will help to hit the right spots for a woman. I reckon that she will get more use from a large penis than a man gets from large breasts.
Exactly. A penis is used for intercourse. Breasts are not. They aren't comparable.
And yes, obviously size matters. If you have a big dick, you have clear physical sexual advantages over guys with average or small dicks. You can reach deeper inside a girl, stretch her out more, and just penetrate her in ways that small/average dick can't. Unless you find a girl who is really tight or shallow, a big dick will give you a sexual advantage that no average/small guy can make up for.