"The banana is fat" wasn't accepted, it only wanted "big".
I've reported it.
In the context of a banana, "thick" would be the correct descriptor. To say a banana is "fat" is at best childish language.
I disagree. Reasonably often I'll hear adults refer to things like "fat bananas" or "a big fat sausage" or something. "Thick" and "fat" would give a slightly different sense to the listener. Fat will often not sound childish then.
Quite often in these comments sections I see people declaring a wording of English to be wrong because they personally, from whichever region they are, are used to hearing a different wording and don't fully think through other wordings that English speakers use, or different senses or feelings in which things can be said. I believe your comment is one of those cases.
Aside from that, "The banana is fat" is simply an accurate, literal translation of the German.
I agree with Alan. "Thick banana" doesn't sound right to my particular Australian English ear. A banana's skin might be thick, but it sounds odd to describe the whole fruit that way. "Fat banana" is fine, and definitely not "childish".
Literal translations are often bad ones. Describing bananas as fat is at best odd and Duo is right to reject it.
Omg. I love learning! Like when I took ASL and we learned the sign for unbrella, and the teacher says "now, dont open snd close the unbrella because it'll mean something different." :D