What does the size of a JavaScript Library say about it?

When people introduce a JavaScript library, they often mention that the size of the library is very small, like "it is small and light weight, xx K minified, in fact is about yy k when gzipped". Does this means a library runs fast or use less memory? Neither. We know that 1k size virus can use up all your machine resource, and make it dead slow, right? What it means the library can be transported to client side faster. Normally the more features a library provide, the bigger the size. You can use profiler (like the one built in Chrome) to collect data when using the library in real life scenario.