Storage needs don't affect pointer size though. To need bigger pointers, you must need an addressable space this big.

That said, how to you make your calculation about DNA's information density? Without counting water, I've arrived at 720g for one “mole of bits”, which is a bunch of orders of magnitude higher than your figure: 2 amino-acids (262g/mole) [1], 2 Deoxyribose (135g/mole each) and 2 Phosphate (94g/mole each).

[1]: and it's the same value be it A-T or G-C, that's interesting