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.