So I'm working on a trilogy, and I just got a comment back from an editor about the first chapter being fairly heavy on backstory.

He's totally right, but it was a fairly conscious thing because I wanted to recount the important points of the first novel.

Do you think it's actually necessary to include these details? Or just strip it back and assume the readers will get it?

I'll admit I went down this route partially due to Harry Potter, as those books tend to start with Harry reflecting on his current situation and what had or hadn't happened for the previous few months in a similar way to what I've done. Of course just being its in Harry Potter it doesn't mean much.