Trying to dry up supply via pumping

I've had to stop nursing my baby directly and I'm trying to dry up my milk supply solely via pumping. I only pumped a little for an occasional bottle before.

I am decreasing the minutes I'm pumping for slowly each day, and my breasts are less engorged so in some ways it seems to be working BUT despite being down to 3 pumps a day, each pump is just getting more volume? I pumped about twice the usual amount in 8 minutes this morning. Is how I'm doing it (decreasing time and pumps) going to work or if the total milk output is staying the same/increasing despite less pumping time, am I going to have to do it some other way - and if so, how!?

I've only been trying this for a week so may just need more time.