Disclaimer – I direct this to no one individual guy.
You don’t. That is rude and mean-spirited. Your opinion may not reflect reality. Some girls ARE heavier (so are some guys), but compared to what, exactly? Our society has been conditioned to think that size 2 is normal for a girl’s body type. In my grandmother’s day, a woman was considered to be most attractive at what we would probably assess as a size 12-14. Stop being so focused on such externals and GO OUT WITH THE GIRL if all else is promising. You might be pleasantly surprised to see that you are attracted to her. And if not, there is no need to specify why. It should be enough to say, I am not interested, though she is a very nice person. If she is truly overweight, SHE KNOWS IT. She doesn’t need some guy to remind her.
And btw, weight comes and goes, and many girls who are stick thin when they are dating HAVE been known to gain a lot of weight after they get married and have some children. Should they be divorced for that? What are we teaching our kids to think is important in life?