I used to pug heavily back in Wrath, Cataclysm, and Pandaria, back where your choices were 10 man and 25 man respectively. Since Flexible came out in Siege of Orgrimmar (and eventually became the bedrock foundation of the 10-30 man styled normal/heroic raids we have today) I've enjoyed filling alllllll the way up to 30 man as well, just for high octane fun, it feels like 40 man, but like, 10 less people. I kind of wish it could go to 40. The sheer chaos would be great.
But my thread is about why 10 mans fell out of style. Back in the aforementioned expansions the content was balanced for 10 people, with an exception for opening week raids for Cataclysm, which beckoned some hotfixes from Blizzard not properly tuning the buff system to accommodate for the lack of players. Nowadays the go-to size is 15 man, 2-4-9, but my preferred raiding style for nearly 6 years has been 2-2-6. Numbers don't lie, 2-4-9 must be mathematically better.
I had a lot of fun back in Hellfire Citadel where one of my favorite things to do each week would be to do 10 man Iskar on heroic with a strange 1-2-7 build, where massive dps would actually eliminate the need for a second tank. It was fun, but nowadays the scale of the raids seems counterintuitive, and less people makes it HARDER, when back in Wrath for instance, 10 man was the much easier version that dropped lower item level loot, Naxx for example dropping 200 loot equal to heroic 5 man loot, with the final boss dropping 213, equal to 25 man normal bosses.
What numbers got changed around to make 10 man bad, and 20 man the go-to size for what Blizzard considers optimal balance for Mythics?