Which method is more efficient overall?

[akrabu]

This is where I got that idea, although I may have misunderstood?

Odd repo size with max compression?

That explanation is incompatible with the implementation. Both auto and max compress every data blob, the difference is only how much effort zstd spends in shrinking the data size. In case some data blob turns out to be incompressible, then it just stores the raw data.