I started killing the Empire lords as soon as I had 20 Khuzait raiders and I just kept killing.
When you're a lone Clan and you execute all of a faction, that factions land all goes semi-randomly to another faction, I suspect Empire factions get priority to get other Empire lands but I don't know how it really works. You can re-load and re-execute and sometimes it will go to a different faction.
Although doing this will make a faction's land and on-paper power double/triple ect..... they don't actually draw in more clans or do anything discernible to get more powerful.
I suppose they have more money but as you might expect, the ruler mostly takes all the stuff and seems to have no way of getting better troops with the money.
If a Ruler owns too many fiefs, you'll get a game freezing glitch when trying to talk to them, because they can't display all their fief in dialogue correctly to brag to you, I don't know how many but it's a lot and only happened after I destroyed about 5 Factions and The ruler of the Southern Empire had all that land to himself. I've reported this and hopefully it will get fixed.
When a faction is low on Clans, if enough time goes by they will take in Clans from other factions and it seems like the game is automatically doing this to balance the remaining factions. It took me a long time to destroy the Southern Empire due to the freezing glitch and so they kept taking equal amounts of Clans from Vlandia and Britannia, making them become extremely weak.
It turns out that if you're a Kingdom, via the main quest, when you finish of a faction via execution you have a chance to automatically take all their lands, but you may have to have destroyed the Empire factions first, or meet some unknown requirement, or it could just be random. WHen I first became a Kingdom I had 1 lone Southern Empire lord to deal with and when I killed him all the land went to Battania, but when I killed Battania off it all went to me surprisingly! Although I entertained the though of letting Derthhert and his family co-exist with me, he promptly declared war on me and so 20 mins later I have the entire map!
So, via execution you can take the whole world with only having to siege 1 property for the main quest! I also learned that forced recruitment is actually much more efficient then normal nice guy style. Every NPC in the world hates me, the world if overrun with endless bandits, there's 0 size glitch minor faction guys wandering around, I get 87K a day in taxes, and I guess this is the end of this play-through!