Hi
You need to look at the document format you are using.
300 pixels per cm uses 6.45 times more pixels for any given physical size than 300 pixels per inch. In most use cases, it is unnecessary.
In addition an 8 bit/channel document needs 3 bytes per pixel, 16 bits/channel needs 6 bytes per pixel and 32 bit floating point needs 12 bytes per pixel. Keep 32 bit for specialist use such as HDR/some 3D render work.
When saving a PSD you can choose, in preferences, whether to use lossless compression on saving which results in a smaller file size on disk, but at the expense of a longer loading time (the document has to be uncompressed). In a similar vein, using the maximise compatibility option, adds to the file size on disk by adding a flattened version of the image within the file, but is more likely to be useable in other applications (e.g. Lightroom uses that flattened image to display it's thumbnails.
I would sort the basic image sizing out as in my first paragraph, then don't worry about file sizes, disk space is relatively cheap. PSDs/PSBs are meant for storing master files with full layers etc for re-editing. If you need a small version to send on, use a flattened copy and save as jpeg/png as appropriate.
Dave
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