I understand your question. I just don't have the answer.

I would suspect that there is not a direct correlation between volume and top diameter for all pots, expecially the decorative pots. Obviously, the depth of a pot will affect the volume.

There is probably a correlation table available for nursery pots. These are made to specific standards.

This might help.

8".........3 QT
10".......5 QT
12".....12 QT
14".....18 QT
16".....22 QT
20".....28 QT
24".....36 QT
30".....72 QT
36".....96 QT

When folks on here refer to the gallon size pots, they are referring to the standard black nursery pots that plants come in.

The pot referred to as a 1 gallon pot, is actually 3qts.

So you kinda have to guess if you're switching to a decorative pot, find something approximately the same volume. You can eyeball it, it doesn't have to be exact.

Okay, well the reason I ask is because I see on Country Gardens and other sites that a plant will be described as being a certain size that lives happily in, say, a 5 gallon container. I always wonder if my trees are in something really too small for them, or if they're in something about right for their size. Not that I'm going up to any huge pots because I don't have the room or the strength, but I'm interested in knowing if I'm on the right track.