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.
Unfortunately, for a long time, POSIX system were uncommon on desktops, and most Unices do not provide a clean way to extend it from userland (though I believe GNU libc does).