Go grew too big (in popularity) to be small. Popular languages tend to grow as the userbase grows. This is natural — the language gets used across a wider range of industries, and the programs being written in the language become larger, more mature and more diverse.
The language maintainers need to balance a small specification or a small implementation versus the users' demand, but in most cases at least some user demands win routinely. This is especially true when the language has just one major implementation. There is a significant cost for making that implementation more complex, but there is a more significant cost for making millions of programs that rely on a single implementation more complex just to make that implementation simpler.
