These have been referred to as "lexicalised diminutives" in the literature. The terms "fossilised/frozen diminutive" also occur in other works.
This paper by Bagasheva-Koleva is specifically about the phenomenon you're describing in Slavic languages.
This dissertation by Katramadou uses the same term to refer to words like Greek κορίτσι (...
My guess is that it started as a kind of joke: Diminish the word "diminutive" by taking away one letter and you get something smaller—the iminutive.
BTW, stacking diminutive endings is not uncommon in colloquial German, you can have up to four of them in an ending like "-ileinelchen" composed of "-i", "-lein", "-el", and "-chen".