Pixelmator doesn't remember/follow window size and always opens in shrunken form.
This happens both on startup and when a new document is opened while the app is already running. The behavior is sporadic with Pixelmator files, but always happens with other file types (e.g. JPG, PNG, etc.).
In the video below, I'm using either ⌘Q to quit the app or selecting "Quit Pixelmator Pro" directly from the menu bar.
I'm using a window manager to resize the app to full screen. Disabling it doesn't change the result.
Video: https://i.imgur.com/2ifjSsH.mp4
MacBook Air M1 (2020)
Ventura 13.2.1 (22D68)
Pixelmator Pro 3.3.1 (Mosaic)