in the Behavior pack, when I try to override the vanilla entity with the same entity with a modified collision box, It doesn't work, although the collision box seems to be in effect, the hitbox doesn't change at all. the specific entity tried is a ravager, with a collision box modified with height 7.2, width 2.0.
I attached the entity JSON file
The file you attached does modify the ravager's collision box. You can see this by having the ravager walk under a tree and noting how high up it breaks the leaves.
The issue you are facing is that ravagers have a hard coded hit box that is different from their collision box. You can either change this or add another hit box (I'm not which way it actually functions) by using the custom_hit_test component. See https://docs.microsoft.com/en-us/minecraft/creator/reference/content/entityreference/examples/entitycomponents/minecraftcomponent_custom_hit_test