It's not that it won't increase in size, it the TIME it will take to increase in size. Not only true of poms, but of all trees put into smaller containers.
You can't grow diameter into a trunk effectively in a container. Pots restrict growth, which is one reason why finished bonsai are put in them--it stops the tree from growing too quickly and coarsely.
The thought isn't that the tree will stop thickening completely, it is that it won't do it with any dramatic results. The process is grow the trunk out in the ground or in a larger growing container to achieve the desired trunk caliper at ground level, chop it back, develop a new apex THEN put it into a "show container."