Normally, they try to talk you into overshooting your AC needs, so you end up with a house that’s the right temperature, but too humid. Strange that they undersold.
I get the sense that across the board, hvac people literally just glance at a house and proclaim the size of system you need. There’s all sorts of calculators online where you can put in all the relevant info, and it will calculate your needs accordingly, but they just ignore that.