How To Find Difference Between 2 Model Demensions

Hello, im trying to figure out how to find difference between 2 models, here is shown http://i.imgur.com/7qNojYS.png?2 where FirstModel(Black) is crossing secondModel(white) border, its Model Chest http://i.imgur.com/8LN8hfG.png?1 (So to be clear), and before FirstModel demensions wosn't crossing second's model, Border, it was off http://i.imgur.com/Yyb8vOC.png?1 - for a little, so i had to fix that by look - manipulating Chest Bone(XYZ), so BlackModel demensions would correspond the white Model, so thats acctualy what it should be.
And so how is it possible to get the value of this difference between 1 model and second model, and to place this value in Chest Bone(XYZ), so to make all this(By look Proccess) automatic.

I hope i didnt make it too much complicated to understand, im trying to figure it out for very long time, i hope someone give me a hand pls!