I think Lucas (Geiger) built the small Fox 22DR first. Done as a side project the first one was even smaller that the final product. The small boost in size to the final design was done to avoid problems with the ejector(?) springs IIRC. Maybe it was the main springs.
You get too small and the trade offs you have to choose from betw power, OD & ID, length, ect get less & less.

The rifle was such a hit they decided to make it available and added the tiny M21 DR as well.
Special order pretty much but quite a few were made up on speculation and brought to shows like Vegas and usually sold.

That's how I recall it anyway. Been a while though..