Gaining

Well joke right back at them.
If they say you're getting too fat reply too fat for my pot holing hobby. Or too fat for my medium sized parachute.

If they say, you're getting rather chubby say Thank you for noticing.

If they say, what are you going to do when you're too fat to walk? Say, I'll phone Just Eat.

Have some pat responses. They'll give up once they're used to you being fat.

I did help one woman with this problem: askdrfeeder.dyndns.org/letters.html#Amanda

Good work Dr Feeder.