The space will reduce congestion (and not being squashed into a sweaty crowd in a narrow corridor is a quality of its own), and space overhead will reduce the risk of airborne viruses spreading. Also, for those prone to claustrophobia, it’s an accessibility issue.

Yes, it could have been done cheaper and smaller, if one wanted to reinforce the late-20th-century ideology that public transport is a bare-bones soup-kitchen service for those too poor to drive, in which case the money saved could be spent on cutting petrol taxes. Though thankfully we have moved on from there as a society.