They should be real close. Your variemces you'll have are with flat top or button top, and protected 18650s are longer due to the added circuitry.
This pic shows the difference in length of the ones I have. I already had several of the brown ones before I bought either light, so assumed I had plenty of spares. Then realized the brown ones are too short for streamlights.
The longer nitecore battery works in the streamlight, but the shorter brown ones do not. They both work in my nitecore light.