Removing a part of a paper during major revision
What is common is for the author to improve the paper as much as possible, hopefully making the reviewers happy. What you suggest makes sense, though so would factoring out the removed material into other formal publications sooner rather than later. 

You could ask the editor, making your case, or you could just resubmit your best version according to your current understanding. I'd include a note as to what you have done along with your reasoning. 

But there is no reason why you can't also prepare the long version in parallel, in case the editor would rather have that.