I definitely think it's due to a slow down of the metabolism, plus the effect of HRT.
I've put on about 1.5+ stone since periods stopped 6 years ago - have always eaten / exercised the same but the weight gain has been steady, year on year. Often I cut down and eat less but it doesn't make any difference whatsoever.
Twice I've stopped HRT, for 3 months each time, and I did lose quite a few pounds - so for my constitution, HRT does have some steroidal effect. However, I don't think if I stayed off HRT I would return to 9.5 stone, body fat distribution is now very different.
I have thunder thighs and bingo wings now, my body shape is totally different, dammit, I hate meno!