Author here. The executables shipped in manyclangs are release builds! The catch is that manyclangs stores object files pre-link. Executables are materialized by relinking after they are extracted with elfshaker.

The stored object files are compiled with -ffunction-sections and -fdata-sections, which ensures that insertions/deletions to the object file only have a local effect (they don't cause relative addresses to change across the whole binary).

As you observe, anything which causes significant non-local changes in the data you store is going to have a negative effect when it comes to compression ratio. This is why we don't store the original executables directly.