Thanks for asking.
Zoom in and out feature of lightbox enables you to view the content of image effectively. An option that you have is to select Original Size under Source > Source Size > Desktop Image Source Type. Here is a recorded screencast. https://cloudup.com/c-Ndkq8gsCp
However, what you are looking for is to remove the zoom in and out feature and let the image load in full screen thereby allowing users to scroll inorder to view the content of image. Unfortunately that will require custom development which falls outside the scope of support we can offer as you are trying to fit a large image into limited screen size. However, I can take this as a feature request and will submit into issue tracker for our developers to take a look.
Thanks for understanding.