D3 RAWs are 14 bit, D70 is 12 bit.

Also, if the D3 was set with 14 bit RAW + JPEG then it will include a ~4MB jpeg, whereas the D70's normal jpegs with raws are only about 1.5MB.

And the final point that makes the difference is that the D70 NEF RAW files are lossy, which saves about 20-40% of size on the RAW sensor data. The D3's NEFs are by default lossless.

http://terrychay.com/blog/article/lossy-raw-compression.shtml