We've already heard that beauty is in the eye of the beholder and no surprisingly so is size. You'd think that it would be easier to define, and with some things that is certainly the case. But not quite the same with bonsai.
Everyone expects ( and not without good reason) that a large is taller than a small and wider than a medium, and when that rule is not upheld some customers can be disappointed ( which I really hate), but the true determination of a tree's "size" when it comes to offering for sale on this site is most often based on what you cannot see- the roots.
Every tree I see comes to me in a growers pot and I move it to a bonsai pot before it is shipped. When I can, I'll do it a few weeks ahead of time, but sometimes that isn't possible. But with lots and lots of practice I'm quite confident that I can transplant a tree without damaging it as long as I've worked with that tree before and know what to expect from the roots. Serissas have super fine, almost hair-like roots. Bougainvilleas have brittle roots and Juniper roots are more woody than anything else.
Root pruning is one of the regular steps of bonsai care and it's possible to leave a tree in the same pot for many years by simply lifting it out of the pot, trimming back the roots and putting it back in the same pot. (simply is probably not the word I should have used as you must exercise care when working with roots).
The first "repot" from a growers pot to the initial bonsai pot is the toughest. You can't take more than 1/3 of the roots at any one time and I much prefer to take less since in addition to a new pot, most of these trees are headed off to a new home. so let's not pile too many stressors on a tree at once.
But it's entirely possible that I can be working with two junipers that are visually the same size and yet once the roots are uncovered one of them will fit in a medium pot and the other must go into a large pot....and so because the part that you don't see is so much better developed on one of the trees, it can be puzzling to understand why one is a medium and the other is a large.
Naturally enough, occasionally I hit a very small tree with root structure that demands an oversized pot, no so much because it is well developed, but often because it has developed a few tap roots with a massive tangle of feeder roots on the ends. These are the trees that on their first repot will ideally have one of those feeders removed in the hope that better roots are developing ( with better care) closer to the base of the tree.