In a way this is a huge question. It's more-or-less asking, "How do I write?" I say this because knowing how much to put in and how much to leave out is such an elemental part of the writing process.
I hope I don't sound sneery or dismissive when I say this, I certainly don't intend to, and have played around with phrasing the above to try to avoid that.
If we bring it back to basics, the purpose of writing a novel is to tell a story. We need a good story-line (plot) with interesting characters we can relate to, and so on.
It's like a lot of things, it's not so much what you've got, it's what you do with it!
Because once you ahve the plot and characters, you have to tell the story. You need to give enough detail that the reader can picture it well and easily, without so much that they get bogged down or bored. It's a balancing act, and how much detail to put in is one of the decisions writers have to make in every moment. (You don't have to get it right in every moment, that's why we have re-writes!)
So the answer is that you need to end up with a story which gives the right amount of information, at the right time, in the right way. If your initial draft has too much info, take it out; likewise if it has too little, put it in.
I think of writing like sculpting. the first draft is like shaping the clay roughly, and each subsequent draft gives finer and finer details. And sometimes I need to smooth away some clay, but sometimes I need to add it. Which of those options, when, how much, and into what shape, are totally depending on where the whole is now compared to where I want it to end up.
So there's no hard and fast answer to the question of whether it's better to add or subtract.
A few other points here - you need to aim for the right word count for your genre, that'll influence whether you're adding or subtracting. Also, as other posters have said, some of us tend to be 'underwriters' and others, 'overwriters'.
It helps too to be aware of your weaknesses. Like Janet, I tend to be too skimpy on descrption in the first draft. (*I* can see the scene perfectly, so have no need for myself to describe it!). So one whole re-write is devoted to adding in snippets of description to make the book come alive for the reader. In this case I'd definitely be adding to the mss.
Hope all this helps.
