Yeah, it does look strange, like a lighting storm in there. I just assumed that they brightened the areas according to how much they changed, which caused the center to look like that.
This one looks like it gets pretty large for a short time... link I guess the 'Spirit' rover is in tornado alley ~ there are quite a few animations at this website with multiple dust devils racing across the plain... link Mars lives.
The big ones are about as wide as a football field the smaller ones about 20-40 feet (similar to earth dust devils). The big one off in the distance looks to be tightly formed which would mean a pretty strong wind ~ seems like it could easily flip over the rover if it got too close.
