Do people only give their seats to weak looking women?

I have noticed that men never give up their seat for me on the train or bus. But they do so for other women. Usually its an older woman such as 40 years + who they give up their seats to. Could it be that they notice that I am a very strong and sturdy, big young woman so they think I can stand on my own?
Do people only give their seats to weak looking women?
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