zgrubienie (not sure what that would be in English)
mostly negative meaning, or in some cases it underlines "greatness" or "big size"
definitely the first option (negative meaning), diminutive is more two-ways (smaller or nicer).
zamczysko(huge castle), król(king) - królisko(big king)
zamczysko - dark, unpleasant castle (with nasty ghosts, for example :)
Take the common Ass for example (not the animal kind...)
dupa - ass (arse, bottom)
dupeńka, dupcia - diminutives
dupsko - pejorative
osioł - ass/donkey
osiołek - diminutive (to underline that it's a nice ass)
oślątko - diminutive (to create "baby" version of the big animal, so a young offspring of donkeys)