First, most ISPs provide more download than upload throughput. For example, my ISP throttles my upload to 1/5th of my download. They do this to try to discourage file-sharing. The corporations which dominate the Internet want to turn the Internet into TV II, where you’re simply a passive observer with your mouth wide open, like a baby bird waiting to have whatever regurgitated pablum they want to feed you vomited down your gaping throat. Why encourage you to participate by giving you ample upload speed?
Secondly, I’m guessing that YouTube probably encodes videos on the fly, as they’re uploaded. They may be throttling your upload to limit the amount of cycles they need to dedicate to you for video processing. YouTube also uses hash-comparison to sniff out copyrighted videos, so they may be throttling you while they hash each chunk of your video and compare against their database.
And lastly, it could be a bottleneck anywhere between you and YouTube. Assuming you’re using some variation of Windows, go to start -> run -> and type “tracert Youtube.com” and see whether one of the servers in between you and them is timing out and dropping your packets into the bitbucket.