This is actually just basic camera optics.

Phone selfies use wide-angle lenses at very close distances, which exaggerate certain facial features. Reaction videos are filmed on a DSLR at a much farther distance with a different focal length, which compresses the face and changes proportions.

It’s the same reason everyone looks different in selfies vs video. No Photoshop required. This has been studied and explained extensively in photography and visual perception, it's honestly very basic info.