Muni's very long platforms at underground stations are largely a coincidence of history. The Muni stations on the Market Street Subway are simply the same length as the BART platforms because they were built at the same time in a single cut-and-cover operation. At 710' BART operates some of the longest transit trains in the world, but due to the construction process making the Muni platforms shorter probably wouldn't have netted significant savings. The underground stations between the Market Street Subway and the Twin Peaks Tunnel, like Castro, were built similarly to the Market Street Subway stations since the length would allow for double-platforming or possibly longer trains - although they are not quite as long (Castro incidentally has an unusual curved platform just because of the practicality of squeezing it in between the older Twin Peaks tunnel and Market St).

Like many surface lightrail systems, the total length of Muni trains is limited by the intersection spacing of the surface streets it operates on, among other factors - the train needs to be able to stop without blocking adjacent intersections.

Full double-platform operation of Muni trains on the Market Street Subway has been on and off over the years with changes in station operations and controls, I believe it is currently on at least at Embarcadero. There have been technical (related to the ATC) and procedural problems (people waiting at the wrong position and having to run) that have made double-platform operation tricky. I also think the gains in efficiency by double-platforming are fairly modest since the throughput of the Market Street Subway has long been limited by signalling considerations.

My Dad used to like telling a story about how when BART first opened, none of the trains had overhead bars because the trains were supposed to run so often nobody was going to have to stand.

It's kind of wild that that wasted space for MUNI underground is not a bigger complaint in the main. My fantasy is that the last 20 years of tunnel computerization for MUNI was going to result in multiple trains per station, or hooking multiple lines together at the entrance of the tunnel (IIRC they did this with the Boeing trains until the Meltdown and Bredas) in order to utilize the whole station length.

Or shrink and simplify the MUNI parts, there's no (apparent to a rube like me) reason MUNI stations have to be the same length as BART stations. Make 'em a single block long with NYC style foyers: attendant, turnstiles, ticket machines. No flowers, no coffee, no Jehovah's Witness tables.

Yeah. BART was meant to be driverless so they could have many frequent trains (possibly with fewer cars) instead of infrequent 10-car trains.

in the earliest days of the BART, I believe I saw the BART train doors open on the wrong side of the train, at the Rockridge station. Also those dumb LED destination signs by DAKtronics lasted decades; some may still be in place.