The size of the brain isn't what counts. It's the number of wrinkles in the brain. To put it into uber-geek terms, it would be like comparing an old laser disc to a blu-ray. The laser disc may be significantly larger, but it certainly can't hold more data. Also you need to look at the source of the study. I'm sorry to say it but the British culture has spawned a lot of the canine obsession that exists in the world today. I'll bet anything if they had conducted the same study in a more cat friendly culture (ie Egypt) the results would probably have been different.
Realistically though it's a cases by case basis. Some dogs are smarter than some cats, and some cats are definitely smarter than some dogs. Case and Point, I have lived next to two neighbors who have dogs. In both cases the dogs are so stupid that they sit in the back yard and bark at anything that moves and do not shut up for hours on end. The only way to improve these mutts' intelligence is to point a 12 gauge shotgun at them and pull the trigger. The people in the study didn't look at things like this. They simply looked at general brain size.