I'm just wondering how a youth sized gun properly fits most adults
It really doesn't have to like we've thought for more typical shotgunning.
IMO, you need a thumbhole stock and an optical sight to make this work well.
With turkey hunting, it's often more about holding your gun "aimed" in position for long periods more than it's about quickly shooting from a non-ready stance. The shorter, lighter guns, especially with a thumbhole stock, are much easier to hold steady for extended periods.
Also, turkey hunting is usually more about shooting accurately than shooting fast.
Personally, I'm shooting an adult-sized 20 ga, but do understand why many have gone to the "youth" models, as they are overall shorter & lighter.