Railcraft increases the maximum storage capacity, while maintaining the exact same storage per block as BC tanks. (16 buckets per block) At the cost of significantly increased resources.
Xycraft tanks increase the maximum storage capacity, but reduce the storage per block. The frame and walls are not counted as part of their capacity, only the air space between those walls. So a 11x11x10 Xycraft tank has the same capacity as a maximum size Railcraft tank at 9x9x8. It does this at a reduced resource cost, similar to the BC tank cost, both of which are effectively free.
Yeah, not seeing the imbalance here. BC tanks are cumbersome because the only way they automatically link is vertically, but a tank buried down to level 5 rising to a normal "base construction" height of 70 can hold 1040 buckets, effectively free of resources. <edit out derp>
That's roughly a 3x3x7 Railcraft tank, or a 5x5x9 Xycraft tank, but you can bury a second, third, fourth, fifth, etc tower of buildcraft tanks for the same cost of "effectively free"