Hallo! I would like to nominate @awesomianist to show that I agree with him. That size shoudnt be an issue as long as one is healthy.
Plus, pure weight (or lack thereof) in itself does not tell the full story of a person's health. A 55kg girl at 15% body fat ratio is much much more "sexy" to me than a 45kg girl with the typical "healthy-looking" 27% body fat ratio. And then there is cardiovascular health, pretty as a girl can look, she is useless if walking up 2 flights of stairs gets her all panting.
Then you have general health as a whole. A sickly girl (and boy) is a total putoff. it shows she doesnt take care of her health..
Dont forget emotional and mental health too! I'm not going anywhere near a psycho and someone with poor mental is detrimental to the people around them.
All in all, when it comes to girls, these are the things i look at. How much does she emphasize on her health rather than looks. All those makeup don't hide your non-existent effort to keep healthy. And regardless of curvy or petite, you don't get my attention if you never worked for that body you have!
This is all of course talking at a superficial level. Since i'm already waaaay out of the game, I'm not "in the market" so to speak. And judging a lady (and gentlemens too) based on looks is dumb and unwise to begin with. I'm much more interested in whats going on in the head, whether its deep with thoughts (great!) or shallow with "ermahgerd did you look at Sheila's picture?"(nah!)"