Using javascript libraries enables development times to be drastically shortened. The drawback is that you are relying on 3rd party code that was not developed with your particular requirements in mind. This results in deploying large files that are costly on bandwidth, memory and processor time.
The Firebug extension for Firefox and Fiddler will both show the download (and uncompressed) size of any components and will allow you to browse the source.
JavaScript should be used to enhance, not provide functionality.
If possible, use the large library to capture requirements then re-write from scratch or use a lightweight
library to provide the same functionality.
Use a modular library where you only need to deploy the routines you are using.
If you review the code and can't understand it, don't deploy it.
Alternative methods for deploying javascript
Progressive Enhancement