The only code that really needs to be duplicated would be the lowest level of userspace code that actually calls into the various operating systems. Things like the guts of "open", "read", "write", etc. Regardless of any particular OS's flavor, they're all going to roughly take in the same arguments and return the same results via some flavor of system call.

Each individual function is probably on the order of a few dozen machine instructions. Let's say there's 30 such functions that are implemented and 7 OSes to support. 36307 is about 7500 instructions. The average instruction length on x86-64 is 2-3 bytes. So maybe it ends up around 20KB of code space for a program that uses every possible operating system feature that's implemented?

I haven't looked at the implementation. I wonder how it selects between OS implementations. Does it switch on every system call, use a function pointer table, or do some sort of clever in-memory code rewriting?