I'd like as technical an answer as possible, since this isn't really a problem more of a curiosity. I've noticed that on most monitors and possible TVs, the size of the screen changes slightly depending on the colors on the screen.
For example, my desktop has a blue background, and this window is mostly white. It I switch between a maxmized notepad and this window, the screen doesn't change, but if I minimize and go back to my desktop, my screen shrinks by about 1-2 pixels on each side (more on the sides, but a bit on the top and bottom) then streches part way back. It is noticably smaller on the background.
It doesn't really bother me, but I'm curious as to why it does this. If it matters, it's a 19" CRT running at 1152 x 864, 85Hz.