what do you say?

Simple.

i) If she is, say nothing.
ii) If she isn't, say so.

<Ackbar>IT'S A TRAP</Ackbar>

Drop and give me 50!

"Oh my god, me too! Is this contagious?!?"

She won't, not even hypothetically.

At least your sight is still ok.

Say "Yes, I like it!", then wobble her tummy with both hands while making blobalobalob noises...

You say:

I like 'em round, and big

Bullshit!

"Nah, just your backside, honey"

That's very vague because it is highly dependent on the type of person she is.

I could say "yeah, you gained a little weight over the holidays" to one of the former girlfriends and everything would be OK, while another would start a shitstorm.

In general though it's best to stay away because there's no best reply to give.

Chuck her a barf bag and say "Binge n Purge Baby!".