CGM File increases in size

I am creating a CGM file. In the past when I have created CGM files they always seem to be smaller than the IDR and definitely smaller than an exported TIF file. I am creating a file now by taking a TIF photograph and then putting a call out on the photograph to a particular part. That is the only thing I am doing to the file. When I save as an IDR the file size is 702 KB when I export the file out as a CGM it grows to 1,757KB. When I export the file out as TIF it is 678 KB. Anyone have an idea what might be causing this or something to look for?

Keith