Large universities often have graduate students who teach the majority of classes. Smaller colleges, on the other hand, focus on the undergraduate experience. You can be sure that a professor will teach your classes, not a teaching assistant (TA). The small class size--particularly in upper-level courses, where there may be fewer than 10 students in the class--allows you to get to know the professor, who can help you to make the right choices in your education.
With smaller classes, it's easier for you to make a difference, whether it's by speaking up in your academic classes or by taking on a leadership position in your extracurricular activities. You'll be able to share your ideas, easily ask questions about things that you don't understand and develop your skills.
With fewer students in your class, the college is able to make your education a more personal experience. Larger universities may have more majors to choose from, but in a smaller school you can design your own major based on your specific interests.
Hands-on opportunities are essential to a good education, especially if you work in the sciences. You need to learn how to use the telescope or how to properly mix chemicals. In a small class, you'll have more opportunities to actually do the work yourself, rather than simply learning about how to do it.