Certainly there is a correlation between muscle mass and strength, but there is more to the story. Two reasons why muscle mass and strength may not be completely congruous are:
- Muscle fiber density
- Muscle utilization
Your muscles are composed of four different types of fibers (slow-twitch, and three forms of fast-twitch). These fibers have different profiles in terms of force applied and recovery time. Slow twitch fibers, for example, recover quickly but have less force as there are fewer muscle fibers per bundle, compared with the fast-twitch fibers.
Extra water in the form of glycogen can also cause muscles to take up more volume with the same amount of actual muscle. This glycogen can be a ready source of energy for the muscles, but isn't going to increase their maximum theoretical force for a single heavy lift (per Olympic competition) where endurance through a long set isn't at issue.
The average person is able to utilize 20-30% of their total theoretical muscle strength when trying their hardest. (Ref. Tsatsouline, Power To The People) Top lifters use perhaps 50% of their theoretical strength. Olympic and powerlifting-style training focuses on training the neural pathways to utilize a greater percentage of the available muscle mass. Since muscle fibers contract all internal cells (the all-or-nothing principal), this training is focused on convincing a greater proportion of fiber bundles to contract during a lift.
Can a buff guy be weak?
Well, it depends on your definition of buff. A cut guy can be weak (compared to a strength athlete), because muscle definition is more about having low body fat covering the muscle than it is about having large muscles.
A bodybuilder with decent volume won't be able to lift as much as a comparable powerlifter because he/she doesn't train for strength per se. It seems worth noting that Olympic/power lifters also want to minimize their size (except for the heavyweights) because it affects their weight class in competition, so there is an added incentive to train for neural utilization over additional muscle mass.