I guess "small" is a relative term. I know my mustang was gathered from Black Rock West. They can freely range from Black Rock West and East giving them 196,000 acres of land and its deemed to be able to maintain 112-186 head.
BTW - if you don't believe they are out there you can go yourself and go see them.
are you referring to branded mustangs or ...? sorry, i'm a bit confused on that last sentence.
I see mustangs every day, multiple times a day - both in the pens at the BLM holding facility as well as off the deck of the house as they mosey on through the property and say hello to our horses. The only ones i've ever seen branded are the ones that were gathered and brought in for adoption. *shrug*