If "malgradna" is opposite of big, that why my solution "The baby is not big" isn't accepted??
I'd say that probably wasn't accepted for two reasons.
1) Malgranda means little. If the sentence were "La bebo ne estas granda," I would expect Duolingo to accept "The baby is not big" as a translation and reject "The baby is little."
2) I don't think that "little" and "not big" are equivalent. If something is little, it logically follows that it is not big, but if something is not big it doesn't follow that it is little. It might be somewhere in the middle.
I'd say the same thing about "Li estas maldika" (He is thin) vs "Li ne estas dika" (He is not fat) and similar situations.
I don't remember exactly where I read it or who said it, or I would give a citation, but I read a quote somewhere either here in the Esperanto Duolingo forum or in the Facebook Esperanto Learners group, that was a good rule of thumb for translating sentences. It was something along the lines of: Be as literal as possible, and as free as necessary. I have found that to be very useful, and I think it applies here as well.