Start by compressing the book proposal into a single sentence with at most 25-35 words.
Yes, you heard me. One sentence, at most 35 words.
See step 1 in "The Snowflake Method For Designing A Novel" for more info.
Here's an example I think you know: A Hobbit must save the world by destroying his magical ring in the fires of Mount Doom.
Is this the Lord of the Rings? No, but it is the spine of it. When we really cut to the bones.
Using this one-sentence summary it wouldn't be hard to compress LotR into a single page. We just need to get rid of a number of subplots, a lot of characters, places, twists, and turns. Maybe not all of them, but the further away from this spine of the story, the more likely it could be removed... when compressing the story into one page.
You should be able to do the same with your book proposal.
Or go one step further and summarize your story in six sentences with the following content:
- The first sentence describes the normal world of the characters before the adventure starts
- Then describe the first catastrophe (the first plot point)
- Describe the mid point, another catastrophe or a more subtle mirror moment, moment of truth, moment of grace (also see here), or all four
- Describe the third plot point / second (or third) disaster
- Describe the climax and the final battle between the protagonist and antagonist
- Describe the resolution, the new normal, the lesson learned, etc.
Later on, you might want to expand the sentences into six paragraphs, but by then you might get more than a pageful...