Boa: 23M Brimstone: 6.3M
I don't know if closing the gap on features with Boa and hardening for production use will also bloat the compilation size. Regardless, for passing 97% of the spec at this size is pretty impressive.
Brimstone does not appear to.
That covers the vast bulk of the difference. The ICU data is about 10.7MB in the source (boa/core/icu_provider) and may grow or shrink by some amount in the compiling.
I'm not saying it's all the difference, just the bulk.
There's a few reasons why svelte little executables with small library backings aren't possible anymore, and it isn't just ambient undefined "bloat". Unicode is a big one. Correct handling of unicode involves megabytes of tables and data that have to live somewhere, whether it's a linked library, compiled in, tables on disks, whatever. If a program touches text and it needs to handle it correctly rather than just passing it through, there's a minimum size for that now.
Disclaimer: I never liked unicode specs.
Any language runtime wanting to provide date/time and string parsing functions needs access to the Unicode database (or something of comparable complexity and size).
Saying "I don't like Unicode" is like saying "I don't like the linguistic diversity in the world": I mean sure, OK, but it's still there and it exists.
Though note that date-time, currency, number, street etc. formatting is not "Unicode" even if provided by ICU: this is similarly defined by POSIX as "locales", anf GNU libc probably has the richest collection of locales outside of ICU.
There are also many non-Unicode collation tables (think phonebook ordering that's different for each country and language): so no good sort() without those either.
FWIW, they are not even "complicated" from a font rendering perspective: they're simple non-combining characters and they are probably never used in ligatures either (though nothing really stops you; just like you can have locale-specific variants with locl tables). It's basically "draw whatever is in a font at this codepoint".
Yes, if you want to call them out based on Unicode names, you need to have them in the database, and there are many of them, so a font needs to have them all, but really, they are the simplest of characters Unicode could have.