Of cos human has the biggest.
And you are kind of right that the bigger the brain, the more clever the animal is. Brain is the place that we store and process the information, hence the bigger it is, the more powerful it is. It is a bit like your computer. However, the body size is not as important. For example, dorphins are very clever, but they have a huge body to go with it. In comtrast, sheep may seem have a better brain v.s. body ratio, but due to their small brain size, sheep are not the brightest bunch=)
Us, it is the homo-sapian or human that has the largest brain in relation to there size, primates (gorillas, apes etc) are next and dolphins follow them. It is normally believed that brain size does indicate cleverness as it give a greater mass within which it is possible to carry out cognitive tasks however there is no substantial proof of this. If we look at other mammals that are believed to be quite intelligent such as the elephant there brain to body size ratio is small. So ideally judging intelligence on brain size is not the best. Hope this helps.