Recent mirrorless cameras are very susceptible to dust, because the sensor is fully exposed when changing the lens. However, many recent mirrorless cameras (Sony, Canon) are full frame.
Is dust less of an issue with larger sensors?
I think this could be the case, because large dust particles won't attach as strongly to the sensor as small dust particles do (contact area is proportional to size squared, whereas mass is proportional to size cubed), and I think large dust particles will be shaken off in an easier manner by the ultrasonic sensor cleaning cycle.
So, the real issue is dust particles that attach strongly to the sensor and won't be shaken off. On a full frame, these smaller dust particles cover less pixels, and with a large aperture, light anyway has an easier way to find its path around the small dust particles.
Googling for "dust particle typical size" finds:
Big particles are considered to be between 2.5 and 10 microns in size, where small particles are 2.5 microns and smaller. 2.0 microns are 50 times smaller than a typical human hair.
A typical full frame pixel is 5.8 microns, whereas a typical crop pixel is 3.8 microns.