Bonjour Joakim !
That's a great question indeed, and one that as a native, I had never even thought of :) So I looked it up, and here's what I found: First of all, this case is specific to the adjective "grand", and here's why: The French adjective "grand" comes from latin "grandis", whose form was identical in both maculine and feminine. That's why French didn't use to make the distinction either, "grand" (ou "gran", ou "grant") was then unchanging in gender. The feminine form "grande" only became widespread in the 16th century. Neverthless, many fixed expressions subsisted, usually because their meaning wasn't literal - "une grand-mère" is not a tall mother! There's a few other occurences, among which "avoir grand-peur", "la grand-route", "une grand-messe", "avoir grand-soif", etc As for agreeing the adjective part of a compound noun, the Académie is still undecided, even though the agreed form for "grands-parents" seems to be the one given by most French dictionaries. I hope that's helpful, and thank you for this interesting question! À bientôt !
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