I have a blizzard survival bag in my 3 day kit. It is indeed about the size of a VHS cassette and probably the same weight!
If you look around you can find reviews or blogs about them. I read one review where a guy got into a 7 degree F walk in freezer in short and a T-shirt. He said the temp inside the blizzard bag got down to 67 degrees after 30 minutes or so.
I tried the thermolite bivy, it seems useful, but i promise you, in a 7 degree freezer the temperature inside that thing would fall far below 67 degrees!
Military and rescue units use this thing (or its blanket-style cousin in the case of evacuating wounded).
link to a blog review:
https://greenbeetlegear.wordpress.com/2010/08/26/blizzard-bag-review/icarus