In the game starting soon, let's suppose I fly to city A with 500 daily demand and city B with 100 daily demand. Assuming all else equal would the size of the demand impact how quickly my load factor reaches decent levels? (assuming im not over 100% demand at either)
Yes. The more demand, the easier it is to fill your aircraft with lowish route image. In your example, supplying 100 seats to the 500-demand-route will prove to be much easier than the other route.