In reply to Gone:
Unsurprisingly, the result in that test is 'bigger flasks stay hot for longer'...
"Why do elephants have Big Ears?" No, not because Noddy won't pay the ransom.
One fundamental problem with stainless flasks over an old-fashioned glass vacuum flask is the metal is a good conductor of heat, so heat conducts from the inner, up to the neck, and down on to the outside of the flask, from where it escapes to the environment. With a glass flask, glass isn't such a good conductor, and the glass vacuum envelope is contained within a protective sheep, often foam insulated, so this conduction problem isn't as significant. Of course, glass vacuum flasks aren't as robust as steel flasks...