correct the chunk size by adding header size

[dangthatsright]

What does this PR do?

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?

thanks @tchaton for the help!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72%. Comparing base (b9aa903) to head (af5db07).

❗ There is a different number of reports uploaded between BASE (b9aa903) and HEAD (af5db07). Click for more details.

HEAD has 12 uploads less than BASE

Flag	BASE (b9aa903)	HEAD (af5db07)
unittests	6	2
3.10	3	1
3.9	3	1
ubuntu-22.04	2	0
macos-13	2	0

Additional details and impacted files

@@         Coverage Diff          @@
##           main   #394    +/-   ##
====================================
- Coverage    78%    72%    -7%     
====================================
  Files        34     34            
  Lines      5037   5039     +2     
====================================
- Hits       3941   3607   -334     
- Misses     1096   1432   +336