[souffle]

I think the vet is wrong and I'm sorry you have had such a bad experience. It isn't normal the way she is behaving. You describe her as feeling sort of large? Does she feel sort of squishy and bloated or solid and chunky? A well bodied young hamster will feel quite 'solid' more like muscle rather than fluidy.
I'm thinking if she feels very bloated that there is something wrong somewhere. There are a few things that can cause this such as a heart defect (these are not uncommon in hamsters) liver or kidney problems or possibly things like tumours in the womb, infections or cysts. To be fair some of these are not treatable and it would be more a case of managing her symptoms to keep her comfy.
Is she chewing the bottle spout at all. Have you seen or heard her doing this?
I'm wondering if her cage is actually too large. I have seen hamsters become very anxious in such large cages and often becoming very reclusive. Big is not always best and sometimes they can feel very exposed in a large cage. Has she always been in here?
Can you give us an idea of your region as we may have someone that can suggest a good vet to see with her.