As an experiment, and to show that there is compression involved even for lossless formats I took an image that I had rendered to Photoshop. I flattened the image and saved it as Photoshop, tif, png and the lossy jpeg. Here’s some screen shots of my Get Info windows showing the various file sizes:
I always use PNG unless I need the Multi-Pass layers….
Look how PNG is half the size of the PSD…. And TIFF is just horrible! Almost TWICE the file size of the PSD.