NTA

I was torn on this at first because what you did was a bit patronising but at the same time by being in denial about her weight gain she's acting childish. Your wife is not a child (I hope) so she should be mature enough to recognise her weight gain herself. By being in denial about it she put you in a catch-22 situation. You were either in for a hard talk where she could snap at you anyway and still remain in denial or you could try and comfort her with a white lie only for her to snap at you when she finds out anyway.

For those that claim OP is TA, I can understand where you're coming from a bit but consider this: As guys we're trained to respond to "Do these jeans make me look fat?" with an automatic "No!" regardless of whether they do or not. That's basically all OP was doing. So he isn't TA but his wife is. She needs to grow up.