I've been working on a DecoNetwork webstore for a client and I'm attempting to implement the mobile-first strategy (starting with the mobile design and adding media queries for larger screens). When I go from the home page to the empty shopping cart page or to a product's information page, everything seems to scale down. Is there a way to prevent this or just contain it to the content portion of the page without affecting the header? I can't seem to find what's causing this either.
7.5 update does not break anything for existing users. The only change we've done is to move the checkout pages to div layout instead of using tables. I suppose if you wrote custom CSS for the table layout you would need to do some re-writing.
As for the original question, the behavior you describe is due to the viewport tag we set. On most pages we set a viewport tag which makes the page scale to the device with. On mobile, certain pages get a different viewport tag set which takes a minimum width into consideration. This was done as some pages have fixed width elements. I think once we move all clients onto 7.5 we can at least remove this behavior from the checkout pages.