Since we are talking about a belt, the natural translation to me was "my belt is wide." Belts have length, width, and thickness as dimensions. "Grosse" seems to properly translate to width, maybe to thickness.
"Big belt" is not meaningless: people do say that they have a big belt, especially in Texas. But I think both translations are valid.
You have to consider what it is that you want Duo to do.
They can choose to give only one meaning for each word. They can ignore the less common but reasonable definitions. But when you get to the general translation exercises you will come across all the common alternatives and more. You will be posting comments about how Duo only ever shows one meaning for a word when there are two or three that are fairly reasonable to be exposed to.
So Duo shows you some alternatives that you might find helpful. But they use the repetitive question and answer questions to train you on the most common and simple meanings of the words.
My guess is that using ceinture to mean waistband in French comes up about as often as waistband comes up in English. Where I'm from that's not very often.
Think of it this way.
Duo gives you some alternative definitions for most words they expose you to. But they drill you on the most common usage.
If you use the less common meanings you will be likely to get more incorrect answers.
They do that because that is what I want them to do. The site seems well used so others must agree.
That's just how it goes sometimes.
let's say someone is learning english, he comes across the word bat, he sees the sentence 'he hits the ball with a bat', then in this sense it's the hitting stick (let's say they call it hitting stick in his native language) that then obviously is another type of bat than in 'a bat is a mammal', let's say they call it 'flying mammal' then it's pretty much clear that for 'a bat is a mammal' they put 'flying mammal' as the highest option and for 'he hits the ball with a bat' they'd put 'hitting stick' as the highest option. same thing here.
It can't. But context, always context and it is for us to think about this as we progress. Gros and Grosse can do Large, Big, Great,Heavy, Fat, Coarse, Thick, Strong, Stout, Full. So, Duo didn't give you all of those options, did it? Spanky Botty Naughty Duo! It is for us to think about just that which we translate as best we can, no?
Girdle has a different meaning than it used to. It used to be a sash or type of belt made of cloth that you wrapped around your waist. Now, though, in English, the word "girdle" is almost always referring to something a woman wears to make her waist/hips smaller (although that fashion item is not very common anymore). The most common translation of ceinture is belt.
I have thought some more about why "My Girdle" won't do and realise that a girdle nowadays only would refer to a horse which is masculine and the sentence began with feminine "MA". I do not think that there is a woman anywhere in their world now, wearing a girdle. I like it... learning a language brings out the fool in me.
DL's translation is wrong. You could see this one coming, the absurd English phrase, followed by our howls of protest when we lose a heart. What idiot at DL framed this question? The correct translation might be something like "My belt is wide" or "Long", but how are we poor souls to tell?
It's very interesting to me that with all of the comments on this sentence over several years, not a single one mentioned what I was expecting to find when I started reading - namely, how odd the audio sounded.... at least to me, this sounded like "ma ceinture est croissant", although it was admittedly more of a g sound than a c sound at the beginning, so I guess "ma ceinture est groissant"