Face crop

A thumbnail on a phone is a couple of hundred pixels wide, and a person shot from the waist up disappears at that size. Cropping in hard, so the head fills a good share of the frame, keeps the expression legible where it counts.

The expression has to be doing a job. A neutral face cropped large is a large neutral face. YouTube's systems hold no preference about how many faces appear in a thumbnail, though viewers do react to one they recognise, so this earns more for an established channel.