It is tempting to use every part of a page while the thought is fresh. You add an explanation, then a second example, then a sentence to make sure the first sentence cannot be misunderstood.
Sometimes the useful part is the first small observation. Leave room around it.
Blank space gives a note somewhere to grow when you return. You can add a date, a different example, or an answer to a question you did not know you were asking.
Leave enough room for the next thing you notice.
A gap also makes it easier to find the note again. You do not have to reread a whole page to recognise the shape of a thought.
Try writing one sentence after a familiar walk. Stop before explaining everything. Come back the next day and see whether there is anything to add.