That means 30 seats less per flight - is the route a poor performer?
I posted the below at #2360 on 30 January shortly after the change to bmi regional appeared in the Brussels Airlines booking engine. It goes some way to answering your question although it obviously can't address the yield on the route.
Brussels Airlines BRS-BRU
The summer timetable is now showing the route operated for Brussels Airlines by bmi regional, still 3 x daily on weekdays. Until the last few days it was shown as a SN ARJ.
bmi regional operated the route for SN before Flybe took it over.
Passenger numbers have risen substantially in recent years from 31,250 in 2009 to 49,046 in 2013, an increase of 17% on 2012. It's to be hoped the smaller bmi regional aircraft won't impact negatively on the passenger growth on this route.