OBS Surprise: "Reset Size" required after changing source's resolution

While fiddling around with Open Broadcaster Software (a very good free program) for recording a screen and a webcam, I experienced something unusual. After manually adjusting the desired resolution of the webcam to be slightly larger than I had it before, the size of the resulting rectangle got smaller and also moved around a bit. I was very confused. After more fiddling around, I figured out that the Reset Size context menu icon on the source made it take on the correct size.