Exactly what Blue said. You see pictures of factory farming with the birds just crammed in there. Most birds, just sitting still, take up a square foot of room! Imagine being packed in a crowd so tight you literally have to bump into your neighbor to scratch yourself....it's horrible.
General rule of thumb for backyard birds is four square feet per bird in the coop, and ten square feet in the run. As WoS said, overcrowding is the number one cause of behavioral issues in birds. They respond by pecking and cannibalism, it's not a pretty picture. Give your birds as much space as you can.
Moonhaus, I agree your birds are WAY too crowded. You need multiple tractors, even 25 birds would be pushing it in an 8x8.
I will make one caveat for what donrae said--the numbers she gives are for laying hens, not for meat birds. Meat birds are generally given less space, and don't have a run. Two square feet per broiler is common. I would give more space for heritage birds, since they are not sedentary like white broilers.
That being said, I find mine grow better with more room. I usually give my broilers and heritage birds (all in the same meat pen) over 5.5 square feet of room, because I can and because they do SO much better with the extra room.