I agree that a 6X2 seems like a big tank as we are used to 4X18s in the hobby but when you think about it a 6X2 only holds a bit more than a bath and we expect a large fish to live out its life in there.
My 6X3X3 seems really small to me now and that holds over 3 times a 6X2 when I include my 5X2.5X2 sump...I am going to swap it in soon for a 6X4X3 or 8X4X3 depending on whether my wife allows me to move a wall or not.
I am so glad I ended up with a Marble Motoro as I really did not appreciate how big a female normal motoro could get until I saw the one in the shop and she looks even bigger face to face!
The thing to note is that her 8X4X2.5 is on a shop system where you have got massive sumps, trickle towers and customers buying bag fulls of other fish so the water is always being changed...Maidenhead Aquatic staff also tend to be very experienced fish keepers with at least one employee or owner at expert level. This sytem has the biggest moving bed sump I have ever seen. I felt that the tank looked too small for her too.