UCI rule is something like a maximum 50cm external total width of the bike.
Drop bars are typically more like 40-42cm centre-centre.
I thought there was a minimum width too (something like 38cm).
Speak to the bike fitters, and they’ll often be replacing bars with a narrower model to get a more comfortable fit. It’s not that 40 is more aero than 44, it that 40 is the “right” size for lots of people.
42cm c-c on my CX bike is perfect
40cm c-c on my roadbike really does feel narrow
It’s only MTBers that have an obsession over bar width, and the right size is one more than what the right size used to be. Roadies have been using the same size since ever, pretty much, give or take 10mm.
edit: but if you’re not competing in UCI sanctioned events, then screw any rules, and fit a wider one if you think it might feel more right.