Neha's cow is bigger than mine - surely this is the best translation, let alone not being accepted!!!
Your sentence is equivalent in meaning, but it's not what's written. The fact that it says मेरी गाय से means that it should be translated as 'than my cow'
Wouldn't that sentence be नेहा की गाय मेरी से बड़ी है। (without the second cow being overtly stated? I assume that construction is OK in Hindi
You raise an interesting point. In English, we have the concept of redundant information. This means that you don't restate something that has been stated. I don't know whether this applies in Hindi but in terms of economy of effort it is likely to apply as well. I guess we need someone well versed in Hindi to enlighten us at this point. My feeling is that you are right:-)