Why does my video playback at different sizes when stacked on top of each other?

It's something that has happened for a while and i'm interested if other people have this on playback when two shots are on top of each other.

So the first shot is what my timeline looks like when static, the second shot is what it looks like when playback is active. I have nested two shots together to use as a multicam.

Once I enable multicam, the shots appear normal upon playback. I'm curious as to the reason why this happens.