Not from my experience. I have a 128 GB iPhone and I only use about 20 GB and my 100 GB Photo library still only takes up 4 GB on my iPhone. I have also noticed in the past that if I take a picture, want to share it about 5 minutes later via WhatsApp for whatever, it is already downloading the photo from the cloud first.
I think it "learns" and only keeps the photos that you access the most.