That is not what a female is supposed to look like. Women are actually supposed to have more body fat than men, and need wider hips for child birth. There is also a correlation between the fat in thighs/hips and baby IQ, as these particular fats provide nourishment to the brain.
If you guys keep going for these gross looking sticks pretty soon we are going to have a sterile society.
That's the scientific part... there's also the plain attractiveness part.
This looks better IMO. Notice how the bottom half is bigger than the top half. Man has known this to be the ideal female body shape for thousands of years - only recently have the apple and ruler shaped bodies been considered "sexy." Big ass/thighs/hips is not "fat," it is just fully developed and big. You can't call a woman fat just because she has a nice rear when her arms are still skinny. That's not fat.
Compounding the issue is the fact that words like "curvy" and "voluptuous" previously used to describe sexy girls has now been stolen by the obese female community.
http://30.media.tumblr.com/tumblr_lexfbucwQB1qg6m06o1_500.png (this one is slightly more risque than the other two, possibly NSFW... no nudity though. It seems worse than it is just because she is so f'king hot)
EDIT: Those pics of Katy Perry probably aren't the best example of a curvy girl, as she isn't very curvy and borders on the "apple" body shape. The other two pics are better, though I can go for even more curvy than those.