my AGENT and I have just gone through this process (from a Blog post on Stages of Editing) up through lines - revisions early on and now just finished line edits. I know that a publisher's editor will edit too, but I'm wondering if my agent only did this because SHE was an editor too? I guess that like many here, I figured a publisher wouldn't buy a book first and then say that major things had to be changed. I didn't pose that as a question, but it is one!
I suspect that agents who were once editors tend to do more editorial work than those who have no previous editing experience. However, no agent in her right mind simply edits or works that hard on a manuscript because she was once an editor. The reason any agent edits or works with an author on a book is to try to eliminate as many reasons for rejection as possible.
Yes, it's very likely your editor will have you go through revisions and edits of some sort and yes, it's possible some of those could be very heavy. However, no editor will buy a book if she thinks it's going to need heavy editing, and that's why an agent works with an author on a book, to eliminate the feeling that the book will need heavy edits.