Analog film photography and cameras

I'm curious, are they super grainy because of poor focusing on my part, the camera (OM2n) or because they are 'Small' scans from FilmDev?

Out of focus images don't produce grain, and the scan size won't make a difference at this size on the forum.
It may be the camera slightly under exposing the images with the scan then lifting the image to correct exposure ?
See if you can compare your cameras light meter with an external one ?
Alternatively, bracket a shot - say shoot the same subject with Portra 400 at 200,400 and 800 ISO and compare ?

I've got an Olympus Trip that slightly under exposes. Shots in direct sunlight with Portra 400 are usually fine, shots in shade or under clouds do have more grain.