Removing a part of a paper during major revision I submitted a paper and got it back with a major revision recommendation. There were many revision comments, and to answer some of them, I added appendices. Now I got the paper back with a second major revision (with many other comments). When I look at the paper as a whole, I see that it has now grown very long. Moreover, some of the results I put in appendices are stand-alone results that can potentially be published separately. I would like to remove these appendices from the paper and make them stand-alone technical reports (e.g. put them on arXiv). This has several advantages: shortening the original paper, increasing the visibility of the side-results, and maybe allowing me to expand them and publish them as a separate paper.

My question is: is it common for an author to do such a thing during the revision process? I understand that I can ask for the editor's opinion, but I would like to know how common is it to ask for such a thing?
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.