If you mean "How do you politely say to a girl that she is quite fat?" DON'T! There isn't a polite way to tell her unless she first asks you for your view and don't forget she probably already knows, so be nice. Also If she's a big girl and can punch her weight she'll flatten you!

"plus-size", "full-figured", "definitely carrying extra pounds (or kilograms)", or, if you're being poetic "juno-esque". It's best not to say anything unless you you feel you have to. And never think you have to remind the woman. There is not a single woman on the planet that doesn't know if she's carrying extra weight. In fact, in the West, many women who are actually slim are worried to death that they are fat. This may be the one of the worst aspects of American culture that we have exported.

Above all don't lose sight of what's really more important: personality.

plump... overweight

Say "You have/She has...(pause) a good body." Everything can be euphemism.