My university was a mixture. For example, the introductory engineering classes you typically take in your first year are large 300+ classes taught by professor

My university was a mixture. For example, the introductory engineering classes you typically take in your first year are large 300+ classes taught by professors with lectures 2-3 times a week. Additionally, TAs (typically grad students) would hold smaller sessions for those classes with maybe 30 students each. Once you move into more specialized classes the size goes down significantly and are all taught by professors.