Oestrogen receptors in the brain control food intake, energy expenditure and body fat distribution. Oestrogen deficiency can increase hunger, slow fat metabolism and encourage central fat gain. As oestrogen levels fall, this turns on an enzyme called ALDH1A1 which causes us to store fat, particularly visceral fat around the internal organs. This is the worst kind because it sends out metabolic signals to the rest of the body! Decreases in testosterone might also account for muscle loss and a 4-5% decrease in metabolic rate.
Hormone replacement therapy shouldn't cause weight gain unless the doses of hormones are far too high for the individual (too much oestrogen can cause bloating, fluid retention and weight gain). In fact some clinical studies have indicated that hormone replacement therapy prevents or reduces weight gain and body fat gain, particularly abdominal fat gain.