[tialaramex]

Two very different things are going on to cause the smaller number of IPv6 routes.

One of course is that some Autonomous Systems don't advertise IPv6, either they have no globally routable IPv6 or they only achieve IPv6 via a tunnel and so that's only advertised via their tunnel provider.

But more important, many Autonomous Systems only need to advertise one prefix in IPv6 because it's big enough. Even if your needs grow, because we'd done this before and because IPv6 addresses are plentiful the allocations were deliberately sparse - so your RIR can give you the adjacent addresses, meaning you still only need one route entry for your larger space.

With IPv4 a provider may find itself advertising hundreds or even sometimes thousands of routes to the same Autonomous System since the addresses they need to advertise aren't contiguous.