That's the small size then. So assuming your wheel is the correct flange to flange spacing of 100mm's and that the axle is definetly seated in the dropouts fully then it is what it is. Although it's tighter than any bike that I can recall seeing.
Something to check is that the dropouts are evenly spaced and that one side doesn't have any extra thick paint preventing the axle from seating to the same depth on both sides. The paint in the U of the dropouts will get chewed up anyway so why not take a moment to carefully scrape it out without doing damage to the metal. With the paint in the curved portion of the U in the dropouts gone I would not be surprised to see the rotor to leg clearance get a little larger.