Hmm. I have several Iris contacts and don't find them that bulky, but they do stick out about as much (plus a bit more for the case) as the CR2 battery they hold. I always thought the Visonic looked bigger, but I guess it's just flatter and sticks out less at the slight expense of other dimensions. I don't doubt they'd look better in many places.
Xiaomi sensors are quite small and I think the Aqara ones look quite nice, but they are non-standard ZigBee devices. Many people get them to work well, but it is a slight risk and does require some extra effort to pair (and stay paired).
I have a few of the Monoprice (non-recessed, PID 24259) and HomeSeer contact sensors, and they are bit large, but perhaps less deep, which may be the concern here. They look identical and are likely the same hardware. These devices also have a "tamper sensor" (small button on the back that is normally pressed in when it's mounted to a solid surface and sends a message to the hub when it gets un-pressed, indiciating the device was probably removed from the door/window/etc.) if that's of any interest.
The ST Multisensor, including the new/2018 version, also is less deep than the Iris, but it's probably taller than most/all of the others (but not as wide as the Monoprice/HomeSeer). It's also a lot cheaper, $20, than it used to be. I found the old generations to work well too, and they're a similar size (but the last gen--was it v4?--was probably a tad smaller than this one), with the exception of the large first-generation/KickStarter-era sensor. ST users have given the last and possibly current gen some bad reviews, but they've been working well for me.