HVAC contractors have been scared of undersizing for years. This is why there are so many oversized systems installed. It all depends on how you define comfort. 78f inside when it's 100f outside is fine to me. I don't want to give up comfort the other 98% of the time to maintain a 72f setpoint on the hottest days. You will most likely need to release the contractor from capacity complaints in order for him to "risk" the smaller system. Your overall comfort over the cooling season will be better with a smaller system.