A bit of a meta-comment: when talking about large standard libraries, people sometimes point to Python as an example of how a large standard library boost adoption (“batteries included”), but one thing people rarely seem to bring up is that it only gained a packaging system fairly late in its lifetime (Python began development in 1991, and the first package-management tool was probably
easy_install.py in 2005). People clamoured to add things to the standard library because despite all the downsides, it was still less painful than trying to distribute packages any other way. And sure enough, as Python’s packaging tools have matured, there’s been less pressure to include things in the standard library (although even today, they’re still less mature than, say, Cargo).
As @aturon says, the important things are the existence and discoverability of high-quality third-party libraries. For Python, reaching that goal involved having a large, standardised collection of packages, but Rust+cargo is in a very different place, and that goal may lie in a very different direction.