Something is off about the integer scaling with the Amiga core. Amiga PAL resolution is 320256/640512. This would mean that a 4x would make it almost fill it vertically in 1080p, considering the usual black borders amiga games have.
I fear retroarch is assuming some of the natural black borders as not being part of the game or image, and using integer scaling forces a tiny game window in the center of the screen. Comparing it to standalone FS-UAE or WinUAE with integer scaling proves that retroarch/p-uae is not scaling the games correctly.