I always figued it was just dependant on the artist's own style, but that fire faerie pic...wow what a difference in the lines!
I work in Illustrator, and you can scale your drawings so that the lines shrink and enlarge so you don't get weird things happening. I've never used Flash, but it must have a feature like that. My guess is that the lineart in some of the drawings is colored in Photoshop, and that would account for the inability to scale properly later on down the line.
I know Disney and all has strict standards for the lines in their drawings...and honestly I enjoy that Neo doesn't do that, at least to any noticeable degree. Part of the fun of Neo is their artwork. Most of the artwork that turns up has Neo's flavor, but also retains a little uniqueness from the artist which makes it a whole lot more like artwork than cartoon drawings.
Hmm, I was just looking at the Polarchucks again before I hit submit, and I realized that they are lacking pressure! Look how even the lines are, they remain the same thickness all the way through. Hmm. Well, they're awesomely cute nonetheless