Imagine a 9" diameter log, to make a bowl from it you first need to split it somewhat off center as the core of the log is often a soft pith.
You now have piece of wood that is maybe 8" across, 4" deep and however long the log is.
You then take a circle template and put the log on a band saw and cut a circle, perpendicular to the grain.
You now have an about 8" diameter bowl blank, sorta flat on the side that was close to the middle of the log and curved back that may have bark attached.
Assume that you are not going to turn a natural edge bowl and lets say you attach a face plate the flat surface of the bowl blank and turn the outside of the bowl. You can either add a tenon that you will clamp in a chuck, turn a mortice that you can insert the chuck into and expand the jaws, or glue on some sacrificial wood to be a tenon.
Since trees are rarely perfectly uniform, by the time you get a nice arc for the outside of the bowl, you are left with maybe a 7" diameter 3" tall partially turned bowl with a small foot / tenon.
Flip the blank over and clamp in in the chuck, turn the face then start to hollow out the inside of the bowl. To ensure that you have enough meat to the bowl, you probably are going to make it no deeper then 2.5"
This is why you end up with a really shallow looking bowl unless you turn it much smaller in diameter and in effect throw away more wood just to get a deeper looking bowl
