In a nutshell, the nursery trade sells trees based on pot size, because the practice is usually correlated with age of stock, and cost invested. In other words, 1 gallon pots until three years, 3 gallon pots until five years, or whatever. It is only the roughest of rough guidelines. You can often find a big chunky tree in a one gallon pot that has been there for a while, or a small tree in a three gallon pot that was just transplanted. I recently bought some landscape deodar cedars that were about 8' tall - and they were all in 3 gallon pots.
I live only a couple of hours away and went to Mr. Maple's Fall open house this year. Let me just say, having viewed their stock first-hand, they try to hit a happy medium with '1 gallon trees'. If the tree is too large to fit in their standard 1 gallon shipping container, they will flag it with yellow tape and set it aside - to be sold local retail but not to be shipped. That is the one benefit to showing up for their open houses - you can pick these "diamonds in the rough" that are too large, or cultivars that they only have one or two of and that they have removed from their site. Of the 11 trees I purchased, only one cultivar was listed on their site as being in stock.
You can see some pics in this thread
I know many of you have purchased trees online from MrMaple.com. Today was their Labor Day Open House, and I was there to check out their trees, snap photos, and say "hi" to a few friends. Here are some pics:
UTV's were available to drive people around the extensive grounds. I think there were 28(?) shade houses, plus extensive outdoor grow areas.
They had an wide selection of larger landscape trees, though the majority of their nursery inventory was in the 1-5 gallon range, which is no surprise since they need to be able to ship the majority of their trees.

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