It's an awkward one really. We moved from state to prep in Y4 and regularly compare notes with parents from the previous school. Some things are very different, other things are not.
In terms of curriculum I don't think state kids are taught less than prep kids for Maths and English by the end of Y6. But in our prep they finished the Y6 curriculum in Y5, and then spent the last term of Y5 and first term of Y6 massively focused on 11+ prep. They've not so much moved on as have just drilled the KS2 curriculum into them. I agree it's not what many would want for their child, but the key is in the name - it's a preparatory school, to prepare them specifically for these exams. What we have heard from our old friends at state is that their kids are all now being drilled for SATs though, so I do wonder how different some of these schools are on Maths and English.
All that said on other subjects, personal development and co-curricular pursuits the difference is honestly as stark as they come, almost incomparable. Maths and English in the end only comes up to about 2 hours of the school day.