No, Magdlyn, they're not myths. They may not be prominent in various areas but Myths they are not. I've spent enough time in the 'sex' lifestyle having conversations and doing studies with men and women to know better than that. Additionally, size doesn't necessarily have anything to do with big ass and tits. I'm a C cup but I'm a bigger woman yet my last girl was a DD and a size 14. My ass has always been full and plump whether I was wearing a size 10 or a size 18. I know some really big women who have, as we like to say, 'noassatall'. Hips and ass are too different things. Additionally, we are making a big generalization. All men are not crazy about tits and/or ass in various degrees.
Personally, I enjoy sex more with bigger women myself because there. There is something more freeing about most of them (not all because my last wasn't all that in bed). I think it is because they have had to spend more time building themselves up and making sure they are on top of their game to get with they want compared to the amount of effort a more physically attractive, 'ideal' weight woman has to exert.