I'm not a fan of the "let's joke about me" strategy but I can understand why some people would like and go for that, my preferred approach aims for neutrality, respect and dignity.
Preventing the situation
You can't control what people think on the general situation, they are simply showing they care about you, but you can change what they think about your particular situation, opportunely and on advance, prior to the situation as you asked.
Tell a story that shows that you have done this before.
If you know you are going to be helping them move things, tell a story in advance about you lifting something heavy. And you can start or end that with something memorable like:
I'm stronger than I look. You'll be surprised.
Even when they haven't seen your arm, that will be memorable. That should be enough.
Defusing the situation
If anyone didn't get the message and they still act condescending by accident, you don't want to be harsh on them, but you are allowed to defend your dignity:
I'm the one helping you, buddy. (smiling of course)
If someone is too condescending it gets annoying, you are allowed to be less amicably, still polite.
It's not a big deal, really, don't worry and don't make other people worry. I can handle myself just the same as you.