Depending on the material, likely strength doesn't allow smaller ones. Regardless, depending on shape and gap width, you can do this with a larger tool, by driving it multiple times over the edge, with the tip in consecutive passes going through paths lying on a quarter-circle of radius equal to tool radius minus desired fillet radius. (if the tool has a flat tip, you need to add that flat tip radius to the path offset.)
Say, the upper-right shows the bit cross-section (well, half of it). With consecutive passes with tip offset at positions on the red line you achieve a fillet of a much narrower radius (blue overlaid by yellow). The only problem is if there are narrow gaps or the depth doesn't allow to lower the tool deep enough.