I just installed Node with npm to use it for frontend dependency management. I know there is also bower but why would I need another package manager that is built on top of another?
When installing a package, npm seems to always load the full source of the js library into the node_modules directory. Just as it's downloading the complete github repository.
How do I install only the minifed (distribution) version of a javascript lib with npm?
There is no standard way to ask npm to get the minified version of a library. Some developers will produce packages that contain both minified and unminified versions (that's what I've done for one of my projects, which is web-only but can be installed through npm) or will create a package that contains an unminified version and another package that contains the minified version. This is done on a case by case basis, varies from package to package, and has to be determined by looking at a project's documentation.
If a developer has not cared to provide a minified code base through npm then you'll have to perform the minification yourself or get the "official" minified code through some other means.