The swelling that goes down in an hour or so may be temporary, but there are changes going on. Sometimes you may not notice the changes, but with time there are changes you will notice such as a bit more fullness or filling your bra cups a bit more.
The swelling itself is a side effect of the vacuum. The growth is caused by the expanded space and stress the cells experience under vacuum. It is typically slow but it is like drops of water into a cup. Over time, you will see the level increase or even overflow if you continue for a long time.