You are correct in saying weight does play a part, and Expro is correct in saying counter rotating brushes will do the job better then a rotary.
However, there are CRB machines which are heavier then others and therefore do a better job.
The heavy ones are Envirodri, Host, Flexi 5 and TM4 (maybe an other 1 or 2 )
the lighter one is the Sebo Duo. Owners like it, but the heavier ones do a better job, and from what I can gather are also easier to clean the the Duo at the end of the job/day.