Brimstone does embed Unicode tables, but a smaller set than Boa embeds: https://github.com/Hans-Halverson/brimstone/tree/master/icu.

Brimstone does try to use the minimal set of Unicode data needed for the language itself. But I imagine much of the difference with Boa is because of Boa's support for the ECMA-402 Internationalization API (https://tc39.es/ecma402/).

Yeah, the majority of the difference is from the Unicode data for Intl along with probably the timezone data for Temporal.