Difference between RAW file size and Photoshop Image size
I have a RAW file from a 24 megapixel camera. The RAW file is about 25MB in size, looking at it in OS X. When I load the RAW into Photoshop and then look at Image > Image Size in CC 2014, Photoshop reports a size of 58MB. I find this dialog useful as it allows me to assess how big I can print at various DPI but I wonder about why the file size has now doubled?
I am guessing that this is because Photoshop has to hold the data in an RGB color space vs the native bayer sensor format of the camera.
Interestingly, when I save the file as a PSD, file size grows to 110MB. I am considering a higher megapixel camera but I shudder to think what the end file sizes will be.
Just as an exercise, I pulled down a 7360 x 4912 RAW from a Sony A7R. RAW file size is 37.7MB. In Photoshop, Image > Image Size reports a file size of 103MB. Saving as a PSD, 254MB.
First of all, size in MB doesn't directly mean anything for print production. Resolution means everything.
When you open raw image in Photoshop, it saves in memory not only raw image itself, but also a copy of the image that you made while import with Camera Raw, so, basically you have original and Camera Raw version of the image in memory. That's why the size is approximately double.
When you save psd file, if you save it with support for previous Photoshop versions, file size can grow up dramatically. So if you don't need support for previous versions of Photoshop, always remember to uncheck that option upon saving.
If you want to keep your files in raw, I suggest you to use Digital Negative (DNG) file format. It's native for Adobe and in the same time uses losless compression.