According to the APPA National Pet Owners Survey, 2009 - 2010, dog veterinary expenditures nearly doubled those of cat for last year.
Dogs are bigger - their food costs more, they generally require more grooming, owners give them more vitamins, they are boarded more often and therefore require more vaccines (e.g. kennel cough vaccine) and surgery costs more.
Here is the APPA table comparing cost of care:
Dogs top line
Cats 2nd line
Surgical Veterinary Visits
Routine Vet Visit