Windows resizing in the background

Since moving to macOS Tahoe, I have a rather annoying issue - a number of my windows will resize themselves to nearly full screen in the background. This is most prevalent on Messages, but also happens under Microsoft Excel and Word. I never see this behavior for the Settings or Finder apps though.

I believe I've turned off the appropriate settings according to the internet but it still occurs. Is this a bug in Tahoe 26.1 or is there perhaps another setting that can be selected?

Thank you!