Everything starts on being brand new or being noob in the market and those current popular sites is no exemption and just take a look on what happened to rollbit?(sorry for the mention). They started small and gradually able to gain reputation until they do made up changes which did really make more interest on them.
It is really just matters on how they would really be handling their business well considering that competition is really high.
Everything needs a process to run and I quite agree with what you say. all things that are good and great will not be big without a process from the smallest first.
depends on how they are managed because if it is good, over time, even small ones will grow and there are many examples and one of them is the one you mentioned.