Ragtaggle, I think you're right. The only person I know in real life who was always offered a seat was very skinny, and hence obviously pregnant, when she was pregnant.
I'm fairly big, and I think some people (particularly men) might not have been sure. DH did used to ask for me, and I think I even asked once or twice. The rule we had was not to ask a specific person. Someone who looks perfectly fit may well need a seat more than you! (Like EIS, they might have knee issues, blood pressure, whatever.) So we would ask a row, at once, IYSWIM.
Normally it was women who got up.
I did have a woman give me a seat from across a carriage, and before I got there, some bloke had taken it. I did ask him to get up. He got up.
Maybe I'm too optimistic, but I think the problem is that people are too busy with their own business and not really paying attention.