I came up with this topic when I was considering units for a Chaos Daemons army I'm planning on taking to the next tournament in Hyvinkää (it'll be a cool lost and the damned style army with ig allies but not necessarily the most stable list to play). I was comparing fiends and seekers. On paper, seekers come with way more attacks, don't suffer from instant death etc.. Most lists would include seekers. However, fiends have the nice advantage of a way smaller foot print (easier to avoid focus fire and hide behind losblock), and can stack more wounds into a smaller area on the battlefield.
Larger squads often have the problem of needing to spread out over a wider area. This was already true in 5th. Squads like Ork boyz (or seekers) rarely got to shoot all their shootas or attack with everybody in combat. In 6th, this has become even more significant due to the new wound allocation system. If you want to be in a position to push a certain way next turn, you need to stack more models there or your opponent's shooting might push you too far back. We in Finland try to always work in a blast or two to force your opponent to spread out. It can have a HUGE impact. My Orks really liked playing against crons, because they rarely had any template weapons. Against anything with blasts, your boys were just less effective.
Smaller squad sizes also don't always mean decreased durability. Multiple wounds, better toughness or saves can make up for that. If you have more durability on one point of the battlefield without needing to stack more models, that can work in your favour. I'd say this is most important with assault units. Losing even one model might increase your charge range drastically with how random charge lengths work. I guess that's one of the reasons 90% of CC units in the meta are multiwound models.
That's not to say that there aren't pros to larger footprints. Some armies play against flyers just by denying effective movement lanes (yes 90 gargoyles, we're looking at you). Sometimes you might need a longer bubblewrap. You might be able to hold one objective and contest another with the same unit. In 5th, when tank shock was the bane of armies like daemons, charging a strung out big daemon unit could block off contesting. But with the way 6th ed wound allocation works, and how a lot of contesters can now move large distances in the shooting phase, there needs to be a good balance. A larger footprint generally means you're giving up your positional strength at a single point, and spreading it thinner amongst a larger area.