I don't know why, but this came to my mind:

I would guess that they're 150×150 because of DeviantArt's archaic system for handling GIFs, where if you want an animated thumbnail then 150×150 is the maximum (although you could just make a smaller version to be the thumbnail instead of making the submission itself tiny, but oh well). Thankfully, they seem to be phasing out this system, but it took them at least 5 years too long.

Another probable reason is that the artist is selling full-size versions as stock footage on other sites, so putting the full-size version unwatermarked on DA would be counterproductive.