The purpose of any file format (and by that I mean JPG, PSD, GIF, TIF, etc) is to store your data and reduce the amount of space the file takes up on your hard drive. Some formats do this at the cost of quality (JPG in particular reduces file size by lowering quality, although not always by a perceptible amount). These are called LOSSY file formats. Other formats, such as PSD or TIFF, don't alter quality at all but don't offer as much savings in terms of file size. All that being said, let's see exactly how much a choice of file format (and a few other things) can affect your file size.

We started with this stock image of a flying cow (how could we not? I mean, he's FLYING!) From there, we saved (from the original each time) copies of the image in various file formats to see how much it affected the file size.

These sizes are with nothing being done to the image at all. The only difference is the container they are stored in.

The other reason for choosing a specific file format is that different formats allow us to retain different amounts of information about our file. For instance, a JPEG can only save a flattened "composite" image ... regardless of whether or not your original file contained layers or other information. A TIFF can save a bit more, being able to save layer information. PDF can technically save every aspect of an image (layers, adjustments, styles, etc), but has to be set up just right to do so. To that end, our tried and true format that we use for ALL of our original, ongoing work files is the native Photoshop format ... PSD. So how much can a PSD vary in size based on your choices? Here are a few examples:

* Used greyscale copy of original image to create layer mask

Again, aside from the transparency (or potential for such) that the layer mask provides, there's not much different about the actual CONTENT in these files. They are all essentially the exact same image. The important thing is to realize that while very few choices in Photoshop are inherently good or bad, they almost all have an impact on your file ... of which file size is one. So do as ye will, but you might need to buy a multi-terabyte drive to do some of it.