By "outgrow" I'm rather sure you mean merely space.
It appears to me (from Oddball's comments) that large fish outgrow their water volume and flow before they outgrow their enclosure space. In other words, the amount of water is crucial not only to their physical movement needs but also to instant dilution of toxins produced by fish.
The instant dilution of the toxins happens before the toxins are taken care of by BB. That's even more important of a reason that a large volume of water is needed to keep a large fish humanely and not have it constantly stressed and suffocated by the regularly spiking toxins, which most usually eludes the water tests.
To further clarify, let me offer a reverse hypothetical example: a 3' RTC in a 300 gal tank with a 5000 GPH flow and a 3,000 gal pond for a sump would probably not suffer from the toxins thanks to high turnover and high water volume but will suffer from inability to move.
Of course, it is far better to provide a 2000-2500 gal tank for this RTC and, say, a 500-1000 gal sump.
As for my standard answer, please look through this collection of pertinent links, if you will:
http://www.planetcatfish.com/forum/viewtopic.php?f=2&t=36395&hilit=+thumb