in answer to my enquiry, I got this mail from Aeromedix :

"The dimension on the Thermo-Lite Bivvy Sack when stored are approx. 6"x3.5"

The difference between the Thermo Lite Bivvy and Heatsheet Bivvy is mainly the material. The Thermo Lite Bivvy is a much more heavy duty item for more serious conditions."
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Alain