I've never understood this question: why is it that in English?
small is the opposite of large
little is the opposite of big
small and little are synonyms
large and big are synonyms
However,
small is NOT the exact opposite of big
little is NOT the exact opposite of large

Question

I've never understood this question: why is it that in English?
small is the opposite of large 
little is the opposite of big 
small and little are synonyms 
large and big are synonyms 
However, 
small is NOT the exact opposite of big 
little is NOT the exact opposite of large

Answer

I wonder how accurate that is. In British English, "big and small", and "little and large" would go together, and almost seem like collocations to me. I would never say small and large, or little and big.

Answer

There is no such thing as an absolute synonym. There will always be some situations where you can use use one word but not the other.You can't learn a language from definitions, translations, synonyms and antonyms - that is why it is important to learn how to use vocabulary in context.

Answer

Let me think and Google. "All things bright and beautiful/All creatures great and small..." ...hmmm... GREAT and small.

"Big and Small: An Animal Opposites Book," 2006 children's picture book title and DOZENS of other children's books

"Big business" is the opposite of "Small business."

I agree with Andi that it's complicated.