Your observations are, presumably, based purely on having printed half-a-dozen photographs during which time you have used nearly three sets of cartridges.
I find that very difficult to believe, especially with a Canon printer.
Been looking at some users' reviews. One states:
"..I have been surprised at how fast the grey ink is consumed printing in color. The overall ink usage, though, is quite frugal. Canon, again, surprises me with another excellent product."
(See: click here)
I haven't found any adverse comments about ink usage apart from the grey cartridge.