actually, it should work that way.
look, when you encode a piece of music at 128kbps that'll mean that each second of the track will "weigh" 16 kilobytes (which are 128 kilobits). at 192kbps each second is 24 kilobytes in size, at 256kpbs it's 32 kilobytes.
sorry, it's the maths. 128kpbs...