Very small aircraft do not make as much money per route. This has two problems, firstly, gate rental is the same regardless of aircraft size (which, come to think of it, is not entirely realistic...), so a greater percentage of your money goes into gate rental. This may actually be a considerable cost if you are only using 19 seater aircraft.
More importantly, however, is that you will not have enough time to create and edit that many routes. Once your airline becomes large, you make so much money each day that you could buy far more aircraft than you could put on routes.
If you are making a DOC of 5,000,000 (which isn't that high, about 25 small aircraft, by your profits), that's 120,000,000 per (real life) day (minus expenses). Earning say, 100 million a day could easily buy you 40 small (around 20 seat) aircraft per day (at 2.5 million). That would mean you would be earning 300 million on the next day (having tripled your fleet), allowing you to buy 120 aircraft the next day... and so on.
While this rapid an expansion is probably not going to happen (you will quickly run out of airports to fly to), you can see the way a problem will emerge. The larger aircraft, costing say 50 million each, might earn 300-500 k per day for the same amount of effort (same number of routes) as it takes to earn 150 k on a small aircraft.
Early on, when you have little money, it is best to buy small planes, but soon you will want to buy more expensive planes to make more money.