Q: why doesn't my iPad mini charge when i plug it into my iMac?

Your USB ports may not supply enough power to charge the iPad. The iPad was not designed to charge from a computer like an iPod or an iPhone. That's why Apple includes a power adapter to plug into a wall socket. The device may be charging at a very slow rate, but you will get a not charging message even if it is actually charging, it will take way too long to charge it that way. Use the adapter and plug into the wall socket.

Read this for more information.

http://www.apple.com/batteries/ipad.html

An iPad, even an iPad Mini, has a larger battery than an iPhone and needs more juice to charge,

Not true.  See this site (I know you said PC, for for others that have an iMac with USB 3 ports, they can put out extra power to charge an iPad.  (http://support.apple.com/kb/HT4049?viewlocale=en_US)

New iMacs with USB 3 ports can up the amps to charge an iPod.