Amazing when a homeowner has more brains than the AC company. The OP
is correct. If the unit were simply too big, it would cool the house
too quickly and it might result in excess humidity making for a cold
damp house. How any AC guy could tell you that your house is 80 deg
because the AC is too big is beyond me.

You don't say where you are, but 6" of attic insulation isn't much for
most areas. I'd look at ways to get more attic insulation in there.
And I'd get another AC company that knows what they are doing. Also,
what is the history on this? Did you just buy it? Did it ever cool,
etc?