Well not only would depend on their roster, but also development philosophy.
Some teams may feel a bigger guy like Byfield doesn’t have much to gain from playing another year against teenagers.
So ultimately a number of factors, and different NHL teams would approach it differently I imagine.
Sometimes it isn't even philosophy. It's market pressure to sell potential improvement. Which is why you will never see a 1st overall not in the NHL in their D+1 anymore. One, the pick will get questioned because someone behind him will likely be in the show, and two, when a team is bad enough to draft 1st generally, all they have to market is the teams future which they showcase through their high pick.
To be honest, I don't think size makes a difference on whether a guy can improve in junior or not. Now, size may make them a more viable NHL player. It is all about what you focus on. Yes, if you want to play a junior game and just do that, then you won't improve, but if you have a list of things to specifically improve on, that you can't experiment with at higher levels because it would be exposed, a lower level is the best place to work on those things.