The answer to this very good question will vary from brand to brand. But most tubs add distance from the front edge of the seat to the floor and keep the seat depth the same.
If that isn't clear, try this - imagine sitting on a good quality desk chair at full height. That is a tall tub - your feel set on the floor in front of you, and you can easily share the footwell with lots of friends.
Now imagine you pull the little lever on the side of that desk chair and it sinks several inches. The back is still the same, but the distance from the seat to the floor decreases. In the tub, this means that you will most likely not be able to put one butt in every seat because the legs get into a tangle.
Of course, most tubs only see one or two people at a time 99 percent of the time, and no matter how deep the footwell area is you tend to stretch out and put your feet up on the seat opposite anyway.
HTH
8-)
