The differences in binary size appear to be caused by enabling split LTO units.
The increased size of .symtab and .strtab are the main contributors along
with some codegen changes. Since split LTO units + unified LTO + full LTO is a
SIE-specific configuration (as discussed above), I’ve re-run the binary
comparison again with split LTO units disabled using compilers with identical
version strings. This setup produced identical binaries.