"I am smaller than you."
Translation:Eu sou menor que você.
As a Brazilian, I say we translate almost the same way you do.
- younger = mais novo/jovem
- smaller = menor
The difference is when we talk about age, we usually use "menor" and "mais pequeno" is wrong in Brazilian Portuguese even when we are talking about size.
Once Duo's course focus on Pt-Br, I think this should be explained.
"Caçula" means "youngest son or daughter" in Brazilian Portuguese. I don't know if it's the same in the European one.
Example: "Tenho três filhos: o Bernardo tem 25 anos, a Carolina tem 19 e o Rogério, 17."
Bernardo is the "primogênito" (but we almost don't use this word, because it's more formal. We prefer to say "meu filho mais velho" = "my eldest son"). Carolina is the "filha do meio" (="middle child") and Rogério is the "caçula" (="youngest child").
More smaller does not work in English as you said. However, much smaller does (as does much less which is related in that minus and menor come from the same Latin root):
1300-50; Middle English Latin, neuter of minor [=] less; see minor
From this same Latin root word we get the English words, minimum, minimal, minimize, and so on. :)