Image file format questions

Here are 2 questions for you guys:

1) Today in my Imaging class, we were taught that 'file size' and 'size on disk' were different because 'file size' is the raw data of the file, and 'size on disk' is the whole size including file header/tags. I always thought it had something to do with clusters, but my teacher was having none of it... which is it?

2) We were also made to do an exercise using the TIFF format for saving indexed colour in decreasing amounts. The decrease in filesize from 256 to 128 colours was rather large as expected, then upon saving in 64, 32, and 16 colours, the file size was pretty much the same as 128. Why is this? Again, the teacher said this is due to lots of tags in the file header. He didn't elaborate, leading me to think he could be wrong.

Thanks in advance, and excuse my ignorance.

>>16

In TIFF parlance it's known as a 'ColorMap', and is referenced by one byte per pixel. Hence the lack of size reduction below 256 colours.

Your two-colour image could potentially be stored as a 'BiLevel' image in the TIFF file, with 1 bit per pixel, if it was black and white. Which would reduce the filesize to approximately an eighth.