It could be a simple matter of adjusting what may already be there (dampener controls). Duct booster fans are also available.
Also available are register fans with thermostatic control (may be all you need).
I wouldn't assume your ducting is undersized.
Are your return vents blocked by furniture? Is the door of your master being left open in the summer?
Rather than assume something was done incorrectly, or that you require larger duct work (which I wouldn't) I'd contact your installer and ask if there are seasonal dampeners installed.
These may be manual at location dampeners (somewhere in the main trunk branches in the basement for example) and by the way if you have a humidification system (for the winter) there may well be a dampener there which should be redirected (to avoid air from your air handler being passed by this (and dragging further) in the summer AC months. There may be cable controled (to a distant enclosed location) dampeners in your system also (they operate using a sheathed cable not unlike an emergency brake cable on your car), these might be handle controled, or a turn screw control, or even electrified servo motor controlled (sometimes interfaced with the "heat/AC" switch on a thermostat.
The heat rises naturally in the duct work, and collects at high points, which is why it requires less powerful fan to assist its (heated air) flow UP to that 2nd floor master bedroom. Cold air sinks...and requires more powerful fan to get it up to that second floor.
Eddys in boxed corners, bends, drags in the ducts that don't compromise that heat flow upwards will overly negatively effect cold air flow up.
It might just be a matter of doing a seasonal balance - for example closing off the registers on the lower floor and fully opening the registers on the upper floor, thus forcing more of the cold air up to the second floor (where you likely have heat gain due to the sun on the roof, and heat elsewhere rising), and keeping a ceiling mounted fan running, then convection will bring the cold air back down to the lower floors.
I'd avoid expecting anything worthwhile from a moderator lately overly cranky, desiring to control and answer every question usually with a great deal of sarcasm and nastiness, and seek help from your original HVAC installation company. It may be something quite simple, or using something you already have.