Мал золотник, да дорог -- lit., "It weighs barely a zolotnik (~4 g), but is expensive/precious" -- thus, "Good things come in small packages"
I'd say "Despite being small, zolotnik is valuable" (zolotniks were used to measure gold, and later there were small gold coins under the same name )

There's an almost forgotten continuation to this saying: "Мал золотник. да дорог, велика Федора, да дура" (Zolotnik is small, but precious, Fedora is big, but foolish), so initially it could have meant something like "Don't judge a book by its cover", but now it's closer to your interpretation.