That sounds very similar to what my Petal had, we didn't notice it until she reached 6 months when her eye started clouding over, once I saw it I knew instantly it was a cataract. We saw the eye specialist, ran lots of tests to see if she would be suitable for surgery (£300) and said exactly the same as your vet said that her eye was actually smaller than it should be, so the cataract op would be a little more tricky than originally thought. Her eye was healthy otherwise, so we went ahead with the surgery. (£2700) before the op she couldn't see out of her right eye but now she has full vision. We took her back for her discharge check up only recently, to be told that some of the cells had been left behind and that 'some' of the cataract has started to grow back......he said we 'could' operate again to remove the cells (at our cost, another £2700 as the whole op would have to be redone) but he said he doesn't think the cataract will grow any more than it already has because basically everything (except a few cells) had been removed from the eye.
So we've left it for now, but it's amazing how much her vision has improved, she catches treats mid-flight which she always missed before. I certainly don't regret spending the money on the op, also because it was something she was born with, no insurance would have covered her.
The condition is called Multi-Ocular Defect......MOD for short.