There are two ways to increase throughput of any metro or train system. Increase the length of the train, or decrease the headway between trains. In this case, The Boring Company is reducing the headways to probably 10-15 seconds.
I'm not aware of any underground (or above ground) system that has a 30 second headway unless you're talking about a gondola which is more close to the capacity that this system can handle.
Yes, trains can hold more, but that's not the point of this system. If you build a 1500 person train, you now have to build the stations and platforms to accommodate those kinds of crowds.
