I took Effexor XR @ 375Mgs a day for 4 years. In that time I gained no weight at all and had not changed my diet a bit.
Effexor is not known for causing weight gain. Maybe some people do experience weight gain with Effexor, but I don't know of any.
I was on Effexor for many years with no weight problem. It acts differently with everyone. If it becomes an issue, ask your doctor to switch you to one without this side effect, but don't just assume that you will gain weight while on it. You're only hearing from a handful of people who have gained weight on it, compared to the thousands who have not.