One of the properties of a good hash algorithm is that a small change in the input makes a big change in the output, and one of the properties of a good encryption algorithm is that the output looks random.

I don't know how the two are supposed to combine, but the prediction is that for a truecrypt disk image, any change in the contents produces a very different disk image all over - so transferring "the delta" is still a much larger amount than you'd expect.

Although, if this were the case it would also have to rewrite the disk image a lot and that might be quite bad for just local use.