I too have noticed this. It's especially obvious if you're using the scale parameter in C4DtoA- past a certain point, it doesn't seem to make any difference at all.
-CMPX
Yes, there's a technical limitation of the radius of 0.001 (in Arnold units). If your Export Scale is 1 meter (1 Arnold unit is 1 meter) and your C4D Project Scale is 1 cm, then there's a factor of 0.01 during export, so 0.1 cm becomes 0.001 unit in Arnold. Changing the Export Scale in the render settings to match the Project Scale might help.
The default export scale in C4D is set to 1 meter, yet it needs to be set to 0.01 meter to honor the internal default dimensions withing c4d. The sss density scaling works fine enough then, so its no bug.