When working on several Photoshop PSD documents that have the same document dimensions (size and resolution) in the same RGB colour space, why do they end up with different file sizes when flattened to a TIFF format with no compression, using the same saving features?
Different areas within a document need different bytes. File dimensions are only one aspect of a file's size.
A pixel that is 0/0/255 needs a different set of data than a pixel which is 199/238/175.
Unless every pixel in your documents match, they will always be different in terms of file size.
all 500x500px flood filled with a color and saved with the exact same steps. The only variation is the actual pixel data. In fact, the only variation is the color of the document. No embedded color profiles, layers, or transparency.
The sizes will vary based on the pixel data.....
Right-click/Control-click and open image in a new tab/window to see it easier
This slight variation, could be due to the embedded preview image for the file. If the preview is using compression, then the different color fields could be compressing differently. For solid, irrefutable, answers as to why this happens, I think you need to ask Adobe as it's a tech support issue, not a design issue.
If you have maximize compatibility set to on then PhotoShop creates a hidden flattened layer.
If you don't need backward compatibility you can turn it off. Choose Edit > Preferences > File Handling.
Change the 'Maximize PSD and PSB File Compatibility' to 'Never' and Photoshop will stop asking you every time you save a new file.
Without knowing the magnitude of the differences you are seeing, it is hard to say. The most likely culprit (aside from metadata differences mentioned by Wrzlprmft in their comment) is a thumbnail/preview which is probably compressed. Depending on the image, these can vary in size. Inclusion of thumbnails is a set-able application preference and also a dialog option for TIFF.
It is plausible that there is some sort of packing algorithm involved when "no compression" is chosen, but I haven't seen any explicit reference to this aside from "pack data into bytes as tightly as possible" in the TIFF docs.