The size you need can change.
My pumping output has decreased some and mostly I have to completely manhandle my breasts to get anything out - but I can hand express just fine. Then I decided to try a 24mm flange - I've been using the 27mm ones since the LC told me they were the right size at the hospital, and the one time I had to use 24mm ones in the first month, those hurt! Now, apparently, 24mm is the right size for me.
I had no idea it could change with time...