Poor image quality when cropping 5.3k footage in iMovie

I'm filming grassroots football (soccer to our American friends) matches in 5.3k resolution at 16:9 aspect ratio on a GoPro Hero 12. I'm filming in 5.3k because I need to crop some of the footage in iMovie, to effectively digitally zoom in on some of the action.

I'm well aware that digital zoom is not optical zoom, so the cropped footage will be at a lower resolution. However, the cropped footage is much lower quality than I expect.

To illustrate, if the cropped area is 30% of the original 5.3k (5427 x 3052 pixels) footage, I would expect the cropped footage to be a resolution of 1809 x 1017 pixels - which isn't that far off 1080p, or Full HD.

However, the footage actually looks pretty much like the players are amorphous blobs of matter moving around in a laboratory petri dish.

I'm using iMovie version 10.4.2 on a 2018 Intel Mac Mini