I'm going to assume you're using MPS for this, correct?
If so, then your problem is with anchors. If you hit the button that says "Show Anchor" it tells you where the image is anchored in the game. Look at the anchor of a normal unit and the unit in your mod. Compare, adjust, and test. It will be a tedious process, but just might be worth it if you're committed.
"We do not quite say that the new is more valuable because it fits in; but its fitting in is a test of its value."