Custom image sizes for Cart and Checkout page images

I'm not sure if I'm really over-engineering a solution, but it looks at the moment as if if I want to customise the image size for products in the checkout and cart pages that I have to replicate the entirety of mp_show_cart() and some of the functions it calls if I don't want to alter core code - which both doesn't seem very DRY, and it also seems to counteract the idea of upgrading - if I'm using customised versions of the functions in my themes then they won't benefit from updates.

Is there another way around this?