If people go up to a queue, and see it spilling on to the Midway, many will skip it. Yeah, there's a sign with a number on it, but we all know how fake those numbers can be. TSL proved that just recently.
The queue should be large, but if you build a queue that can hold 3 hours of people, which for Disney's awful FP+ setup, isn't really that many, you can bet you'll have a 3 hour line. Build it for 2 hours, and you most likely won't have an hour line on the Midway.
I also don't understand why Disne, and Universal for Potter, hasn't started adding bathrooms in the queues. They claim how family oriented they are, but clearly have no clue the average size of a 5 year old's bladder. Or a 40 year olds that is drinking enough water not to die in the Florida heat.