Home Heat/AC Vent leaking?
When an AC is installed that is bigger than needed the discharge air drops considerably, and you may have a 25 degree temperature drop - and 20 degrees is typically considered the max.
The unit will typically short cycle as well. Sounds like something to look into given that you didn't have the issue with your old unit...
A humidifier is typically energized by the control board, or by a current sensing relay that is attached to the low (heating) speed of the blower motor. If it was installed properly, the humidifier can only operate if the furnace is on.
Also, the colder the air is, the more likely moisture is to condense on the diffuser. No different than water droplets that form on a cold soda or beer can in the summer. To minimize this, ensure that insulation is present between the diffuser and the hot air from the attic. Ensure your filter is clean. If it is dirty your AC will move less air, and therefor the discharge air will be colder and more likely to cause condensation. Lastly, ensure the cooling speed on the blower motor is set for the highest speed. If it is set for med-high or low, you will not move as much air across your evaporator coil, and the discharge air will be colder, and more likely to be below the dew point, so moisture droplets will form.