It seems to be exactly like in German: A girl (or boy) is called groot especially by adults who haven't seen a child or adolescent for a while. I believe English doesn't have an exact equivalent, but little as in little child/girl/boy is the exact opposite. It means taller, older, riper (mentally and physically) at the same time. You can't say it really means one or another of these adjectives any more than you can say that a little child really is primarily a small child, or a young child, or one who still behaves like a young child.
I have the impression that it's much less used that way in English, though this may depend on the exact variety or my impression may be wrong due to limited experience dealing with families in English. But in the specific phrases big brother/sister we obviously have precisely this usage.
KaiEngle already explained it above. What you describe only applies to common gender nouns (in Flemish: masculine or feminine). But meisje is neuter gender because it's a diminutive.
PS: This wasn't a complete explanation. The exception is only for neutral gender, singular nouns without a definite article. But this is the situation we have here.
Difficulty of pronunciation does not necessarily depend on number of consecutive consonants. I know Russian and I can tell that none of those 4-consonant clusters is harder than the "voiced velar fricative + voiced uvular fricative/trill" cluster. Now I choose replace the Dutch R's with alveolar trill, which is a lot easier to pronounce.