In Ukrainian, 'яйко', 'яйце', 'яєчко' ('yaiko', 'yaitse', 'yayechko' - a small egg) are also used to describe an egg. 'яйо' ('yayo') or 'jajo' in Polish is what little children use in speaking. See the similarities?
To me "jajko" sounds like it's meant to mark a small egg - what would be the actual diminutive of "jajko"?
"Jajeczko". The opposite would be "jajo" etc., But not used very often or only for certain situations
I am wondering if in polish we can use the "egg" word when we talk about man's organs (balls) . Because in Ukraine and in Russia the eggs (яйца) also means man's balls.