Physics collision of attached static meshes (things act as if no mass)

I have a bunch of physics objects which, when just pushed with the character capsule collider, respond more or less as I want them to. They have different mass, etc. But if I attach a staticmesh to my character (just a cube for now) and collide it with one of these physics objects they act as if they have no mass at all. They can easily be tumbled around and smacked away with this attached cube. What am I doing wrong?

So to clarify, I have a normal mannequin character with a cube attached and floating 2m in front of the character (attached to the capsule). I have two cubes in the world, one weighing 1kg and the other 1000000kg. The character capsule can push the 1kg one but not the heavier, while the cube can easily push either without any issue at all. Surely this is just some setting I’ve missed? I’ve attached it with AttachComponentToComponent and tried both weld on and off without any luck. What am I missing here?