Hi Toddy, the large cups are very stable when full and their sides and handles do not lose rigidity when filled with a hot liquid.
Heat insulation by the plastic is good allowing the cup to be cupped in the hand with very hot liquids.
A folded small cup will fit into a large one easily and they will then fit into a Snowpeak 700 Ti cup with the minimum of squeezing, with a little bit more force they will fit into a 600 Ti cup.
A useful feature of the large cup is 100ml graduation lines inside it.
http://www.renajs.se/
Happy trails...torc.