The meaning is opposite. When you say "Zece nu e puțin" you mean that in fact it is a big number. Example: "yesterday we had 10 virus cases" - "Zece nu e puțin! Ia în considerare că toate erau raportate de același spital" (Ten is not a small numer! Take into consideration that all were reported from the same hospital)
Though Romanian is a romance language (which I'm greatly enjoying learning) I'm often intrigued by how it's subtly different to the others.
I'm just curious; have the numbers been influenced by the Slavic languages around the region, does anyone know?' Zece', 'sapte' and 'opt' in particular, sound like they might have been, though they are obviously still related to the Latin equivalents.