So the solution to MY problem was as follows.

My footer image was ~1200x60px, so I resized it down to about 600x30px and it looks exactly the same on all the pages now. I think the problem is that TCPDF wasn't resizing/scaling the image in the footer properly, so it was putting the image in the ORIGINAL size on the PDF. Now that the image is saved to the actual size of the generated PDF [width], it looks exactly like I need!

The issue still remains that TCPDF does this, so it's good to know how to "locally" resolve this incase anyone else has this problem like I did.