How come some guys are strong but not very big?

Strength and muscle size are not necessarily connected. Getting muscles to grow, called hypertrophy, is the result of lifting moderate to heavy loads with moderate repetitions (6-12) and higher volumes (sets). Additionally, diet and recovery need to be conducive to that end. This approach causes micro-damage to muscle fibers and as the body recovers it seeks to repair the damaged fibers, making them thicker or larger to handle the stress better in the future. Maximal strength is a different physiologic adaptation. When exposed to extremely heavy loads, the nervous system adapts over time by learning how to activate more of the muscle fibers immediately so that the body can handle the crushing weight. Think of it this way: hypertrophy or size is a structural adaptation whereas strength is a nervous system adaptation. If one constantly trains with moderate loads and high volume and eats a diet with enough calories to allow growth, they will eventually increase their muscle size. They will likely get stronger over time as well, but not at a rate that matches one who is specifically training for strength. Training with very heavy loads that are 85%-100% of your 1 repetition maximum and longer rest intervals (2-5 minutes) to allow full strength recovery will cause the nervous system to learn how to recruit more muscle fibers immediately and keep them focused on the task at hand. Muscle size gains may be minimal with this type of training. The key to being both big and strong is to vary your style of training and include both variable groups. Just be sure not to neglect your foundation. If you have poor muscle control or form, then these types of training can quickly amplify these issues and lead to injury. Also, when training with very heavy loads, have a spotter to help in the event your muscles fail and you cannot complete a repetition.