In an English campaign of mine, some time ago, he had conquered northern Italy (he took Napels, but failed to hold on to it), and expanded into the Balkan, taking (iirc) Hungary, Croatia, Greece, possibly also Bulgaria, but my memory is a bit dim here, and even Constantinople

I held the rest of Asia Minor, as well as the provinces to his north, as far east as Austria (Hungary had originally been conquered by myself, but I ravaged it and left again). Since he was my faithful ally, I was pretty much the only reason he didn't expand further.

Apart from my own experience there, I've also heard tales of the Papacy conquering Wessex, for example.
In the HRE campaign I'm playing now, however, he's reduced to the Papal State. He shouldn't have gotten in my way like that.