pusto - bardziej pusto, najbardziej pusto (puściej, najpuściej)
staro - starzej, najstarzej
stromo - bardziej stromo, najbardziej stromo (stromiej, najstromiej)
The alternate comparative forms for pusto
that Hiro gives may be analogous to English forms:My glass is emptier than the others,
although it isn't ungrammatical, sounds less correct than My glass is more empty than the others.
But the English equivalent of Michal's suggestion (simply use more
) simply doesn't work:My glass is smaller than the others
is correct, while My glass is more small than the others
sounds very wrong.
As others have indicated, such a simplification doesn't always work in Polish either.
Many thanks for the responses.