TL;DR; The VS2010 ISO is small because it excludes MSDN documentation, which is not yet available offline; users can access documentation online instead.

The reason is simple, it has no MSDN documentation in it. I for one never install the MSDN documentation and just use the online version and have always found it a pain to have it inside the same installation, but may people find it useful.
And, why is there no Offline MSDN documentation? The answer is simple, it is not ready yet…
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