I've been running into the same issue for the past few months. Based on what I've found (your FeedValidator helped me to figure it out), it's actually an issue with Blogger. It turns out that when you copy&paste/drag&drop an image directly into the Blogger editor, the image is converted automatically to base64. This means now you have a potentially HUGE RSS feed, because of said image.
I'm not sure how to solve it from FWP. The only answer I've found is to make sure that authors don't include images via copy&paste/drag&drop.