Fiber Optic Cable Capacity

I am trying to design a system that will carry light for illumination along a fiber optic cable. I see that there are many thicknesses of fiber optic cable - however I am unsure of the purpose of this. I've never heard of a 'density' of light that you can put into a cable - is there such a thing? Ie. is there a certain 'amount' of light that you can put through a certain diameter of fiber optic cabling, kind of like there is a maximum amperage you can put through a certain diameter wire? (I know it's due to resistance in electrical wire, which is very minimal in fiber.. which would lead to a negatory answer)

If not, then why the different thicknesses of fiber? (I'm talking about single strands for simplicity)

Thanks!