The preview is probably showing you the jpeg thumbnail inside the raw file. When you Open the file you see the actual raw data.
Two things might be happening while looking at the actual raw data:
Affinity Photo displays more data around the edges of the sensor than other programs do, giving you more data (a larger picture) than the pixel dimensions that your camera maker advertised. This is intentional, and the Serif developers expect you to crop that data if you decide it is not useful to you.
You have some vignetting that you need to remove. A lens adjustment in the Develop Persona may handle that, if Photo recognizes your lens. Or the Remove Lens Vignette adjustment may do it.
Or it may be a combination of the two.
