Bone, Do you post process with Photoshop?
What format are you delivering to them?
I know if you open a .JPG in Photoshop and "save As" in native Photoshop .PSD, the file can be up to ten times the size of the original .JPG.
I just opened a 173KB .JPG pic in Photoshop, saved it as a .PSD, it is now at 1,266KB with no editing.