If Remote Device -> Acquisition Control -> Exposure Mode is set to “Off”, the exposure time is automatically adjusted to maintain the maximum frame rate.
When the resolution is lowered, the frame rate increases, meaning the time per frame becomes shorter. As a result, the exposure time is reduced and the image appears darker.

How to keep brightness fixed:
Set Remote Device -> Acquisition Control -> Exposure Mode to “Timed” (fixed).
With this setting, brightness is determined by the configured Exposure Time and will remain constant even if you lower the resolution.