In smaller glass tanks, the bottom is often "floating" or inside the vertical panes due to silicone's higher tensil strength. On large glass tanks, it is not uncommon to have the vertical panes on top of the bottom and another internal "eurobrace" to take advantage of the tensil strength of silicone.
With acrylic tanks, the vertical panes are usually on top of the bottom, this is mostly due to the fact that it's *much* easier to get the tank to sit flat if the vertical panels are on top. If the bottom was "within" the vertical panels, any warpage (at all) in the bottom would cause the tank to not sit flat causing undue stress on the bottom seams. During gluing, the simple wight of the tank will flatten out the bottom, we almost always glue the concave side up for just this reason.
James