Its a complicated question. Within the human species, the correlation is not that strong at all. In cross-species comparisons it tends to be quite strong, especially when considering brain to body weight ratios. Bird brains may be an interesting exception however, which I suspect can be quite intelligent, depending on how intelligence is measured.
Part of what makes that interesting is we are very good at some tasks that might be quite expensive, and really bad at some tasks that could be simple. In other words, our personal views on difficulty could be wildly off. For example assuming computer vision should be easy and chess hard.
