The routes grow a little over time, but in the life span of Beginner Worlds you will not go from filling up a B737 to Filling up a B747 if that's what you are wondering.
Another reason for the demand differences from real life (which I think) is,
In real life flights between LHR and JFK are often operating as HUBS, passengers gathering in LHR from different points in Europe and flyting a A380 to JFK then taking smaller aircrafts further to their destination etc... while in real life scenario you have many transit pax, you only have point to point traffic in Airway Sim.
I have tried to create HUB routes with Airway Sim, it does not work. I opened up a base in Oslo, and tried to gather traffic from various airports in Norway to Oslo, and then fly a bigger aircraft to a long away destination,.. although traffic from BGO-BKK and TRD-BKK, RYG-BKK etc all together should fill up a large flight, in reality it only showed passengers that was the demand from OSL-BKK.