Poor quality after rendering in Sony Vegas?

I use my iPhone 4G to record some videos. The video quality is very good. But when it's time to render this. MOV video to. MP4, the quality drops (Sony Vegas Pro 10)! This happened only today. The previous days I could render videos with quality loss 5%. Today, the quality loss is 30%. Any ideas what could have go wrong so badly?

Go watch this video and use the settings he uses. Then render your video.

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