Is it the Volume of Water or Size of tank that matters?

[Ridewithme38]

I'm on the very first stage of considering a larger tank for my fish, but the issue is i have a very specific area i can keep it in that limits the footprint i can use...

This tank will end up being for two fancy Goldfish, a Redtop Oranda and a Black Moor, I have an AquaClear 110(500gph), TopFin 30(150gph), 3" SpongeFilter and 4" Spongefilter running in my current tank, turning over my tank water about 24x an hour...i have read that length of tank is more important then depth for Goldfish, but i'm wondering does volume of tank matter?

Basicly the Largest i can go footprint wise would be 37wide by 13 deep....

Within this footprint, there are:

Gallons.......................Width x depth x height
26 Flatback..............36 1/4 x 12 1/2 x 16 5/8
23 Long...................36 1/4 x 12 5/8 x 13
30 Gallon.................36 1/4 x 12 5/8 x 16 3/4
38 Gallon.................36 1/4 x 12 5/8 x 19 3/4
45 Gallon.................36 1/4 x 12 5/8 x 23 3/4

Given "perfect Filtration", why would the 45 gallon be better then say the 23 Long? What benefit does deeper water or more volume of water have on Goldfish when you have REALLY REALLY good filtration?