The wider the wand vacuum slot the more vac power is needed to recover the water you put through the carpet. Hence TM's can cope with wider wands than the lower powered porty. Whatever type of extractor you have it's all about the balance between solution psi/ pump flow and vacuum power/ water lift/air flow.
Tms can operate at higher psi and bigger wand jets because they have masses of vacuum power to recover the water.
IMO porties should have no more then a 12" wide wand, with twin 110015 jets, whilst TM's can get away with wands 15" wide and multi jets.
If you are using a very wide wand with a porty you will be leaving the carpet wetter, worse still if it has large jets as well.
Dave.