You just need to modify the y origin of your layer at the same time that you modify the height. I don't know exactly what your code looks like, but if you're taking the old frame and applying a constant change to the width and height, like this:
CGRect newFrame = oldFrame;
newFrame.size.width += kWidthChange;
newFrame.size.height += kHeightChange;
_myLayer.frame = newFrame;
then you just need one more line, like:
newFrame.origin.y -= kHeightChange;
before you reassign the layer's frame.