Let's start with what is a developmental editor? This type of editor - which differs highly from a line editor or proof reader - looks at big picture elements of your story.
1. Does it hold up from beginning to end? 2. Is it coherent? 3. Do you have the necessary foundation in place to execute the story? 4. Are there holes/details left unexplored?
1. How do they change/why? 2. Are their actions well supported by their motivations? 3. Does the arc you planned make sense and have the necessary stepping stones? 4. Are all of your characters needed?
1. How fast/slow are you introducing it to readers? 2. Are critical details missing? 3. Does the world make sense? 4. What tools do you use to make it feel authentic?
1. Is the story dragging or moving too quickly? 2. Is there enough time to adjust to changes/information given to us? 3. Are you focusing on something you shouldn't be? 4. How is this affected by sentence structure?
Once I've read/annotated a manuscript, I then address issues via an edit letter. These vary in length but I want to make sure authors understand the issues and know where to go from my feedback.
The point of a developmental edit is to make sure the pieces of your story move together like a well oiled machine. A story needs various types of structures and with even one off center, the machine begins to break.
Concerns pointed out by a developmental editor aren't fixed by changing a sentence or two. They require diving back in and finding how you can stitch your story together.
Whether that's adding a chapter, removing a secondary character, creating a new subplot, rebuilding your world, etc - every story will vary in what's needed and how much is needed.