What to do when reviewers want you to expand on introduction and discussion but you have space limitation?
"Expand" doesn't necessarily mean add words. You can add material to your intro yet still condense the manuscript by making other edits somewhere else. 

Sentences can often be restructured to save on word counts. I've managed to cut hundreds of words from papers simply by revising and editing. Sometimes you can eliminate words or phrases by using langauge that is more direct and concise. Do this right, and you can reduce your word count while still making your paper more forceful and substantive at the same time. 

It might take some time and effort, but have the right mindset. Don't assume your paper is currently as concise and polished as it can be – assume there is room for improvement.