No matter how you stretch it....

Life size model. 

Miniature shot Model

The two models of the Jupiter two above were  designed and used for exterior shots of Lost in Space.   The curved extended lines of the smaller model at least give the illusion of a lower deck.which had not been thought of or included in the original script when the series was planned. Although still physically impossible to fit.


And let's not even think about where they'd fit the third level reactor core, seen only in one particular spooky episode (and a particular favorite of mine.)