That's how the Hukou system works, there are two tiers: Farm/Country and Urban, and they want to urbanize farmlands, which will upgrade existing farmland hukou to city ones.
They are not talking about moving farmers into existing cities.
> What they want is urbanization of the less developed areas
Everybody wants that. Can't remember that any country ever got it, in the last century, anyway. That's akin to wanting water to roll uphill.
Argentine will prefer its population to not clutter around Buenos Aires. France will prefer everybody not move to Paris, same for Russia with Moscow. But that's just not what is happening anywhere.
If you're moving to a city, makes total sense to pick a powerful one.
USSR did it to some extent. By forced work assignments after university in bum-fuck nowhere. As well as scarce goods more available in those cities as a bonus.
On top of that, people were not allowed to freely move. You couldn't just move to a city, rent apartment and look for a job. For people in countryside, university and assignment after it was pretty much the only way to move to a city. University courses were highly limited though and far from everybody could get in.
USSR tried to do it to some extent, still Moscow grew from ~1.5mln to 8mln on their watch.
The key was that while theoretically the state wanted it to stop bulging, a huge number of influental state enterprises wanted more workers and wanted them now, and got their way more often than not.
And Russia still deals with millions of people who are spread around evenly across landscape in single-employer towns where there's no longer any jobs. And no economic reason for these towns to have any jobs in the future. Some of these in the far north or Siberia for no apparent reason.
Who said they tried to put a hard cap on Moscow at some lower number? Moscow grew to 8mln precisely because of government policies. Had they no limits, it could have been 2x that.
And yes, that policy is definitely baiting ex-USSR countries in the ass nowadays.
The fun thing with autocrats, they want everybody to be on short leash, on the distance of calling before eyes or local phone call.
So naturally, when they say they want to cap population of main cities - doesn't mean they're ready to delegate anything essential to 2nd tier cities! And they consider many many things essential being control freaks, so that's where you get additional growth.
E.g. in the USSR, the main filmmakers were in Moscow and SPb. Could you relocate them to e.g. Sochi as it happened with Hollywood in the USA? Sure, but would you be able to hold them as tightly? Hence they stay.
What they want is urbanization of the less developed areas, many of which were farmlands from before.