I waited to answer because it's been a long time since I took microbiology and what I learned is mostly ancient history by now- keep that in mind. The infectionness of organisms depends on size, ability to survive outside host and ability to replicate in new one.

To me, seems easiest to understand by looking at hospitals' use of different isolation precautions for different types of infections. Airborne precautions are used for diseases whose organisms are so tiny, they can remain suspended in air for some time after a patient coughs and another person across the room person can breath in the particles and become infected. It's used for highly contagious airborne organisms. Patients with measles, chicken pox and TB (mycobacterium tuberculosis) get these precautions.

Droplet precautions is another category. The organisms are larger and heavier and soon fall to surfaces close to patient when patient coughs. It's used for influenza, diphtheria, and others. It's important to mask if in close contact and to clean the surfaces near patient. It's recommended patients be masked when they can tolerate it.
Both TB and MAC are both mycobacteria, but are different in size and spread.