Problem with bounding box when mounted

Hi.
There seems to be something strange going on when mounting player object to player. What I did was mounted a player object to a horse object.
Which mounts just fine but the thing is that it can only move on flat ground or it will get stuck.
I don't know why it (or they) get stuck but I think it's issue with the bounding boxes, because I noticed that using the same size for boundingBox for the human fixes the problem!

So the boxes are like this:
Human: boundingBox = "1.2 1.2 4.0";
Horse: boundingBox = "2.0 4.0 4.8";

Which doesn't work, the horse gets stuck easily on slopes and can move only on flat terrain.

However if you use boundingBox = "2.0 4.0 4.8"; for both human and horse, the horse wont get stuck any more.


Any ideas what could be wrong?

Thanks!