Many four year schools did get bigger, and many were founded in the post-WW2 (GI Bill) era. For example, the state of California founded 14 (out of the current 23) CSUs and 5 (out of the current 9) UCs with undergraduate programs after WW2.
However, the increase in capacity may not have kept up with the increase in enrollment demand since the 1970s or 1980s, as indicated by increasingly difficult admission thresholds for what were once easy to get into four year schools.