Iron tanks don't count internal volume, they count all blocks the tank occupies. Xycraft only counts internal volume, by comparison.
A Railcraft iron tank's capacity is <number of blocks occupied> * 16 buckets; it's based on the capacity you would have if you filled the same 3D space with Buildcraft glass tanks. Therefore, your 3x3x8 tanks have a capacity of 3x3x8x16 = 1,152 buckets, and your large 9x9x8 tank has a capacity of 9x9x8x16 = 10,368 buckets.
The newer steel tanks do <number of blocks occupied> * 32 buckets. So if you need more room in your overflow tank, maybe swapping it for a steel one would be a way?