I don't know literally a single thing about the PDB format, but assuming it at least somewhat similar to DWARF, it's REALLY easy to get an explosion of symbols

I don't know literally a single thing about the PDB format, but assuming it at least somewhat similar to DWARF, it's REALLY easy to get an explosion of symbols in there, especially if you're looking at individual object files. A clean checkout of chromium is 5 GB (excluding the .git folder) by itself, the symbols will of course be larger.