How to properly generate Compound Collider?

So, I have a rigged Humanoid character with no inbuilt animations or animator, and I plan to move it using Physics. I've already painstakingly coded the joint rotations, movement restrictions and sensitivity to Physics, now I need a Compound Collider for the experimental method i'm using. I saw someone build one for a gun a few days ago:

But when i try to build a Collider for each child i find that each Collider (Capsule Colliders if that's important) could not be made smaller than a certain size, when i try to make it smaller it expands out the wrong way and severely bugs out the hitbox of the Humanoid. The smallest size i can get is an oversized circle so far. Is there a way to tweak the collider size of an object so it fits the Child object perfectly?