Need advice on usual procedures with HVAC companies in replacement

I need professional advice on a situation I am dealing with. I know not every company is the same but I feel I am dealing with possible “bad business”. We moved into our home and 2 years later our AC/pump died. We had a company come out to replace with a new one and extra proper ductwork we needed. We are now 4 years out from this new unit being installed. We have always noticed it seems to run constantly but we live in the south. We have had a few times it just couldn’t keep up and we had service. As of recent the service did not help much. The technician said what’s your square footage, I think your unit is too small. Come to find out it seems the new unit installed was too small for our house and along with the region we live in. No one ever checked that out. The company says you need a bigger unit and it’s very pricey and here we are left with a Carrier that still has a lot of life left and 5-6 years of a warranty on it. The company says it is not their fault they just go by what they removed and didn’t consider anything else. Is this normal for companies to not consider square footage or any other factors and just assume?? Are we screwed in this matter? I know nothing in regards to this but I am trying to learn more. I feel it could be bad business but I just don’t know. All I know is we are not sure how we will afford this new unit. Appreciate any feedback!!

source https://hvac-talk.com/vbb/threads/2238985-Need-advice-on-usual-procedures-with-HVAC-companies-in-replacement?goto=newpost