I am not endorsing your numbers since I haven't done the math myself, but assuming you're correct, it wouldn't be unusual to see 75k support in a city in mid- and late-game in an active world. Say, 4k biremes and 2.5k FTS? Definitely possible.
Remember, city population puts a cap on the forces you can recruit and send out in a command, but there is no limit to the amount of support that can be stationed in a city.