correct the chunk size by adding header size

[tchaton]

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

This adds the chunk header to get the proper file size.

Fixes #393

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in GitHub issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 78%. Comparing base (b9aa903) to head (124ee8f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #395   +/-   ##
===================================
  Coverage    78%    78%           
===================================
  Files        34     34           
  Lines      5037   5040    +3     
===================================
+ Hits       3941   3944    +3     
  Misses     1096   1096