Agreeing with the others. You don't have to answer "How much do you weigh?" by anyone. It's your business alone. Bean dip that question.
If it was anyone else, yeah, but if my parents asking I'd probably answer.
However, I'm not saying everyone should be obligated to. Just that I can understand why the OP didn't bean dip, because I probably wouldn't expect that question to turn into an exchange like that either if it was my mom doing the asking.
I'd answer my mom if she asked how much I weighed. I probably wouldn't even think twice about answering. Though I can't think of time she's asked in recent years.