I think you will always find something that the other product doesn't have (or doesn't have in the same way)
Postgres e.g. has the infrastructure for extendable data types and index types and the infrastructure for extensions - none of that can be found in MySQL and that infrastructure needs source code as well.
I don't think the nearly 3-fold difference can't be attributed to "features", but to other things. Maybe the projects are structured differently or include different additional tools in their code (e.g. the input files to generate the SQL parser, rather than the generated parser's C code).