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.

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