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Google admits it's making YouTube worse for ad block users
(www.theregister.com)
This is a most excellent place for technology news and articles.
Someone above posted an article saying they aren't actually. But you'd be surprised at how little space those 10 hour videos can actually take. They're highly compressible since they're just the same still image and the same audio on repeat. A good compression algorithm (which Google certainly is using) would basically compress it into one instance of the song and how many times to repeat it (more complex than that, but that's the idea)
Sometimes they are, if it's just audio and a static image. Some of them definitely are not that though. The ones with visualizers or full music videos or the like are not nearly as compressible.