looking well?

[Snog]

when people say this do they mean you have put on weight?

I would say it when someone looks tanned, fit, healthy...?

No!
I have never meant, you've put on weight when I've said it to people. I'd say it to someone if their skin, hair, clothes looked good.

i mean it as a genuine compliment if i say it but have been told by several people it means you look fat!!!