"Why does a bra maintain swelling?" (Sorry I could directly quote, this thing keeps saying ten or less smileys when I only put 1! Now I have none and it still told me the same thing!)
It actually makes sense to me when it comes to the swelling staying longer when a bra is worn right after.
I'm guessing the pumping brings more blood and bodily fluids to the breasts. When the pumping is finished, the fluids eventually return to their normal levels. With a bra, the pressure given by the bra AROUND the breasts slows this process and "holds" the swelling longer.
That's just my idea.