Q:If I plant a gallon-sized Leyland cypress, will it grow more quickly and match the size of five-gallon plant in a relatively short period of time?
A: I think the answer is “It depends.” If the smaller plant has an immature root system, having only been in the pot a few months, the bigger one will grow larger faster. On the other hand, if the larger shrub is root-bound in its pot, it will slowly take off and might stay the same size for a couple of years while a healthy smaller one zooms past it.
You’ll have to pull the plants out of their pots to compare the root systems. If the root mass seems to fill the pot without circling and doesn’t have a hard, dense mat of roots inside the pot enclosure, that’s a healthy plant. If two healthy plants of different sizes are planted side-by-side, the larger one will be the winner for a few years. But after growing for five to ten years I doubt you’ll be able to spot much difference.