In general, a Mac does not control the brightness of an external monitor as such.
The explanations regarding min/max values in GPU kernel extensions and PWM signaling to LCD drivers chips are incorrect. That sort of thing applies only to the internal display on the laptop.
In your case, you have an external monitor connected over HDMI. There's no PWM brightness signal in a HDMI connector - it simply doesn't work that way.
Only some monitors actually support controlling their brightness from the computer - and again in some cases it's a proprietary interface. In general, external monitors that do support brightness control over HDMI, do so via DDC.
DDC is an I2C bus with a somewhat standardised, digital protocol on top that allows for information to flow to/from the monitor and the computer. The brightness "signal" is thus not analog voltages or a PWM signal, it's digital commands.
On top of DDC, there's a standard set of commands known as MCCS (Monitor Control Command Set). A few monitors also make MCCS available over USB (for example the old Apple Cinema HD display).
MacOS does not send out these MCCS commands over DDC to external monitors. Apple has their own system for managing brightness on their own external monitors, but it is not used with third-party external monitors.
To sum that part up - macOS is not asking the monitor to lower its brightness.
This means that there are two possibilities:
- The settings in the on-screen-display menu on the monitor has the brightness somehow set lower than maximum (unlikely given the information you have included in your answer)
- The Mac is not sending the same image data to the monitor to display as the Windows PC.
The latter can happen because of color profiles:
On the Mac, check System Settings > Displays and "Colour Profile". Try changing that to see if it changes the brightness output, you're measuring. Browsers can and will use colour profiles - so even if you display the same PNG file (for example) in two different browsers they can show different colours - even when the colour code is listed as #FFFFFF for example. The browser might apply a color profile as can the operating system in general.
If you want to try setting the monitor brightness from macOS, you can do that by installing a third party DDC utility. I think the best/easiest is to install BetterDisplay or MonitorControl.
It could also be for example the
ddcctl tool available from HomeBrew, however I'm not sure how well it supports your M1-based Mac yet.
For the M1 specifically there's another tool named m1ddc. Unfortunately it is not clear whether that works when controlling a monitor connected over HDMI. So you might need to use a different cable to connect the monitor over DisplayPort instead.