It goes like this:
The desirable physical condition is slim and fit. That is undeniably the most attractive.
Fatness is not attractive, except for chubby chasers, who are most likely themselves chubby or fat.
THere is a heirarchy. Attractive people go with attractive people fat people with fat, ugly with ugly. That is natural selection at work right there.
If you want to mate with an attractive woman, you must work for it. Prove yourself worthy of passing on your genes. However, you may naturally be cursed with ugliness. At least you can work on your body then. Or you may be intelligent and a naturally handsome face, but fat. This would necessitate your working out a lot, as the first thing women notice about you is your physical appearance, and they will judge you for it.
Etc., etc.