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.
RAW formats differ quite a bit even within manufacturer.
Canon CR2 for example appears to use anywhere from 2 to 4 greyscale "lossless jpeg" (non-linear quantization?) channels to encode the data. It is not stored as RGB in the manner of a typical TIFF or JPEG, nor are the channels themselves necessarily the same size.
Even though they may be storing in floating point or 12-16 bit integer formats (i.e. more storage), the packing and encoding methods used can still account for the smaller file sizes you are describing.
reference: Understanding What is stored in a Canon RAW .CR2 file, How and Why (IFD#3 is the RAW portion)