On large files, allow for additive size reporting.

Failing to pass False triggered _update_hashes to overwrite the size.

Example, the file size would be built like:
0 + 4096
0 + 4096
0 + 337

resulting in the package being reported as 337 bytes.

Fix:
0 + 4096
4096 + 4096
8192 + 337

being reported as 8529 bytes.
Merge pull request #3 from grahamc/patch-2

On large files, allow for additive size reporting.