Well according to the wiki the capacity of the small 5x5x5 is 160.000L.
But then the 3x3x3 one should have 160.000 / (5x5x5) * (3x3x3) = 34.560, which makes yours pretty close to being realistic even though you tried to make Darth's mod even more unbalanced by uping it. I think his one has a 80.000 capacity as he said he halved the original.