Windows growing over time

[eightmeow]

I size my windows so that they are just small enough that the main video takes the full width of the window. And then I make it a little smaller than that. This will work for a day, maybe a couple days, but eventually it will start sizing the windows larger than the size I set, and the layout of the window changes to allow thumbnails up the side. To be sure I wasn't imagining it, or there's not something else about the layout that is causing this, I screen-capped it immediately after setting it to the size I want, and a couple days later when it is no longer working I can compare, and I can confirm that the window is wider than it was when I set it up.

What is causing this?