Circles collision inside circle

[cxminer]

I'm using Phaser with P2 physics and I have a big circle in which I have many small circles. How do I make the small circles collide inside the big circle and never go outside the bounds of the big circle? Any ideas?

I found an example here https://phaser.io/examples/v2/p2-physics/collide-custom-bounds but instead of a rectangle I want it to be a circle.