Well, I've read that sizes vary from one brush maker to the next. I guess I've got a good example on my hands here.
I've written previously that I felt like I needed something smaller than my Rosemary & Co. 3/0 spotter. Since that's the smallest she makes in that line I went with a Da Vinci 5/0 extra short spotter that I found on Amazon. It's a pretty dramatic difference visually:
The picture was taken under the 6x magnifier lens on my old man lamp. Part of the difference may be due to the Da Vinci being an 'extra short' brush and thus missing much of the 'belly' that the Rosemary & Co has in spades. I'll try them out tonight and see what I think, but I suspect I may still something in between.
EDIT: In order to present a more 'apples to apples' comparison, I cleaned and pointed both brushes. Here's the after shot: