"The quantity is small."
Translation:La kvanto estas malgranda.
Why is 'eta' wrong?
I believe that "eta" doesn't really mean small, but more to a small extent. For instance "Cxi tiu homo estas ete granda" (or cxi tiu homo estas grandeta) would mean "This man is a little tall". It doesn't mean that this man is both tall and small, but just that he is tall to a small extent. You can do the same with other adjectives: "La ovo estas varmeta / ete varma" is "The egg is hot to a small exent", or "The egg is warm". I hope this clears it up for you. :)
Nope. Eta means "small". Ete could mean "to a small extent", though, I believe.
Etymology of kvanto
From English quantity, French quantité, etc., ultimately from Latin quantitas.