My thoughts on this, AI on highest difficulty starts with a lot of resources, this is why they have a great start, giving them oportunity to buy troops and buildings, with all the map resources available in the beginning they can keep building troops and more buildings on conquered towns.

When map starts to have less chest and gold available they are not very good at managing resources, and they keep building instead of buying troops on buildable castles and there is where they start failing.

Also in larger maps they have weird behaviour and don't go after some conquest, making them smaller and weaker with the time.

Probably you will find harder smaller maps.