I'm a newbie, so take what I write with a grain of salt. My understanding is that narrower tires reduce rolling resistence. Lower rolling resistence means higher speed (all else being equal). A narrower tire is narrower and has a higher PSI rating, so there is less surface area in contact with the ground. So the basic idea is narrower tires = faster.

The trade-off is a harsher ride. Maybe less traction in certain situations? Maybe higher chance of a flat? Not sure about those last two.

BTW, I believe the 7.3FX comes with 32' tires while the 7.2 has 35'. Road bikes often have 23' or 25' or 28'.