OP also posted https://blog.jakspeedruns.com/opengoal-project-update-septem... a while back. It has a section on "How is this possible?"
TL;DR for folks is that they left a lot of debug information in the build. The C code was compiled with no optimizations (and included debugging symbols), and GOAL uses a string-based table for global functions/variables/types, so they're starting with all those for free. Plus they don't think the GOAL compiler did too much tricky-to-undo optimization either.