Big or little, the first week or so of having freedom offered, they tend to stick very close to the run. If you have small chicks, and you're not sitting out there with them the first few times the run door is left open for them, I'd probably loop some chicken wire around to somewhat contain them, just so they can baby step their way to freedom without worrying as much about cats and such. That way they THINK they have freedom, but it's restricted...
It took a long time for mine to venture away from it. I thought they'd never let it out of sight. Then, one day, I noticed they had followed me as I was walking to another part of the property - and that was all it took!
I still have a group that likes to remain close to the coop....