Although new underground style trains have recently been introduced in some parts Sydney (a really stupid idea in my humble opinion considering the distances that they're expected to cross), I don't have firsthand experience of them because they aren't in the parts of Sydney that I travel to. However I have of course had experience of metro like systems in Rome and Milan. London carriages are noticeably narrower. Obviously seating is relatively rare (compared to heavy rail) in an underground system, but in this case there is relatively little standing room as well. My suspicion is that that's because the trips are generally expected to be short and the services frequent, so if there is no standing room on one train you just wait for the next one. If there is relatively little space in the carriage, it lessens the chance of people needing to climb over each other to get in and out meaning that a more frequent timetable can be implemented.