“Did you put on just a little weight?” Don’t even imply this.
“Is the weighing machine working right? It says I am lighter these days.” Not unless you want to trigger her disproportionate concern on the correctness of the scale’s report about her.
“Carol still looks good, doesn’t she?” Especially if Carol was your ex. Even if she wasn’t, refrain from uttering anything like this anyway.
And the ultimate one:
“These can’t be my kids.” Never ever say this, even when your kids are performing way better than you did. Or even when you mean it as a compliment to her family.
Of course, it is perfectly OK for your wife to say the same. That’s the way life is.