I'm overweight, and I'd be really upset and offended if you said this to me. I think it's a classic case of 'if you can't say anything nice, don't say anything at all!!'
After all - what good is it going to do? I bet she's well aware how much she weighs and doesn't need to be made to feel bad by someone who is, presumeably, slimmer than she is!!
To be frank, I think it would be a thoroughly nasty thing to say.