Every time you minimize and then un-minimize the lobby, it grows in size a little bit. If you do it many times, it will eventually grow quite large.
Here are some screenshots depicting this. Open them in separate browser tabs and then switch between the tabs to easily see the difference:
Freshly-opened lobby
After minimizing/un-minimizing it about 30 times
It’s not a huge deal (doesn’t impact gameplay), but when I have it running all day and I’m constantly minimizing and un-minimizing windows, it not being a consistent size can get a tad annoying.