> I imagine the BigInt index that points to the starting digit for your data would take more space than the data itself

Assuming optimal conditions (i.e. optimal randomness), you can expect that as many pieces of data will have pointers smaller than them, as pointers bigger than them; you can't win that way necessarily, only in some cases.

But maybe if you can find two 'opposite' algorithms, such that for any data where the pointer is larger with the first, it's smaller in the second...

(From what I know of information theory, the extra bit you have to use to specify which algorithm to use will outweigh all savings, but it's still fun to think about.)