there's something funny going on all right
my full screen shots are about 3MB
They are same size whether 8 Bit or True Colour - actually I dont think that's funny - just that your "TIFF's" are so small ...
My ancient Photoshop 5.5 cant open the 8 Bit or the larger true colour TIFs which surprises me cause I usually work with TIF scans from various sources and have no problems
maybe I just need to reboot ..
Photofiltre has no problem with either
both programmes can save a much smaller TIFF version than S.C. (in Pshop if LZW compression is selected, in P'filtre via default save)
the problem with opening the tiff seems to be with the system here somehow (now having problems with other tiffs with another programme)