Force is changing shape?

I'm new with Unity. I'm trying to create simple game. I already created my "hero" that is orange voxel cube. That hero got cube collider (shown on screen). I also got obstacles (the red one) also made of voxels. I use force on "hero" so he is going forward and all his voxels are moving together. But I see that collision doesn't work how I want and I don't know why. I used force of 3000 and then my hero and obstacle collided even though such a collision was not visible (like on screen). I changed force and then whole situation changed (without moving objects!), collision happend but now its also visible. And here is my question, why is that happens? Or what informations can I give to you so you can diagnose the problem.

I want to move "hero" with force of 3000 and calmly move past obstacles. Before i made my hero a voxel hero (he was just one cube) there was no such a problem.