I'm wondering what happens when the author and the agent differ on what the book should be. For example, what if the agent suggests a large cut to the work, almost a quarter of the book, but fails to specify what exactly should be cut. As an author, I would rather have the manuscript pitched as is or broken in two books. Not that my prose is golden, but I strongly feel that cutting the manuscript would leave it crippled.
Let me guess. Your manuscript is more than 160,000 words?
Big fat manuscripts give me the hives.
Unless you're writing in the fantasy genre, they're also MUCH harder to sell.
Your agent, in telling you to cut, is giving you advice about improving his/her ability to sell this tome. In telling you to make the choice of where, s/he is respecting your ability to make tough choices.
You, in your reluctance, are thinking art not commerce.
I am an agent, thus commerce wins 9 out of 10 falls. My thinking is this: it doesn't matter if it's great art -no one will read it if I can't sell it.
I've had this exact conversation with potential clients. I won't take anything that's much over 100,000 words for a first novel. The plain truth is I have lousy lucky with big fat novels like that.
Maybe other agents can do it. I haven't.
In the end, this is your book. If you can't stand to perform a pageectomy on it, you need to either: find an an agent who will take it as is; write another, smaller, book; or, publish it yourself and prove everyone wrong.