The biggest factor in MQMs ability to drag in more people? Karachi. It is a city much more massive than all other Pakistani cities combined and the areas MQM holds have the highest concentration of people in Karachi.
So yes, MQM does have much more "crowd pulling" capacity on call than any other political party.
However, the largest crowds that dwarfed the spurious claims of "large" at the PTI rally were seen much before. First in the rallies of Bhutto, then against Bhutto once when Karachi rebelled against him in the 70s.. then at the return of Benazir back in the 80s and recently, the "true" crowds pulled by the MQM back in the 80s.. and so on. Compared to them, this PTI gathering is much smaller.