If you run the same bus route and frequency, one day with buses that fit 50 people and have 100% utilization, and the next day with buses that fit 100 people and have 100% utilization, you transported twice as many people on the second day as on the first day

This implies that on the first day a lot of people either didn't make their trip because there was no space, or took a different mode of transport. Presumably mostly the latter. With the highway example the same is true, just that it will be mostly people not making the trip at all before the capacity improvement because they were unwilling to make the higher time commitment