IIRC from working with Brotli before it's not that the it's truly a "different dictionary" but rather more like a "reverse view" of what is ultimately the same dictionary mappings.
On one hand it seems a bit silly to worry about ~100 KB in browser for what will probably, on average, save more than that in upload/download the first time it is used. On the other hand "it's just a few hundred KB" each release for a few hundred releases ends up being a lot of cruft you can't remove without breaking old stuff. On the third hand coming out of our head... it's not like Chrome has been against shipping more for functionality for features they'd like to impose on users even if users don't actually want them anyways so what are small ones users can actually benefit from against that.
On one hand it seems a bit silly to worry about ~100 KB in browser for what will probably, on average, save more than that in upload/download the first time it is used. On the other hand "it's just a few hundred KB" each release for a few hundred releases ends up being a lot of cruft you can't remove without breaking old stuff. On the third hand coming out of our head... it's not like Chrome has been against shipping more for functionality for features they'd like to impose on users even if users don't actually want them anyways so what are small ones users can actually benefit from against that.