Ohhh, so that's why … How to Tame Sequoia’s Window Tiling - TidBITS:
Have windows on your Mac started to expand to fill the screen since you upgraded to macOS 15 Sequoia? … having windows change size underneath your pointer can be jarring. Luckily, new settings in Sequoia let you move windows around your Mac without having them jump around.

thanks this is useful. I’ve been very confused by this feature and was considering disabling it.