John shares his knowledge of fiber optics by answering your specific questions
Can 125µm fiber be used in a 126µm ferrule? Why are the holes so close in size?
Generally, the recommended ferrule hole size for 125µm singlemode fiber is 126µm. The 126µm hole allows for some epoxy bond around the fiber, and for ease of fiber insertion without breakage. However, there are some 125.5µm ferrules available for critical alignment situations.