Papa C nailed the reasons perfectly. There is nothing more I can add other than if you go from 2.5 tons to 3.5 tons, the refrigerant lines, especially the suction line, might have to be replaced. If I was your HVAC company, there is no way I would increase the tonnage of your system unless a residential load calculation (Manual J) said your home needed the extra tonnage. Doing so, maybe not so much in a drier climate like yours where there is much more sensible heat (versus latent heat), could lead to, what I call, a “sick” home. Not only that, it would be a waste of money to increase the tonnage if your home doesn’t need it.

As a rule A/C systems require 400 CFM of air per ton of capacity. So your current 3 ton unit is probably providing 1200 CFM of air. The evaporator coil needs adequate air so that it won’t ice up. That means that the fan has to be designed to provide that amount of air, and the ductwork has to be designed to distribute it. A certain size of duct can only handle a certain amount of air. If it’s too small, there will be too much pressure on the duct, which can lead to air leaks on the seams, etc. Think about what water does when you partially block the end of a hose with your thumb. You have increased pressure and velocity from the end of the hose. The same principle applies to air, with the added factor of noisy vents and drafts. That’s why your HVAC tech mentioned that to you. It’s possible that your folks home had some oversized ductwork to begin with which was why an upgrade to their A/C was possible. A good HVAC company should be able to provide a load calculation for you to determine how much tonnage you really need, along with a duct calculation to provide you with the correct sized duct and required amount of registers.

Peter is right. Another problem with a larger tonnage is that too much cool air is coming out and the system will not cool evenly. That’s more of a problem where the humidity is higher. Just something to consider.

Short answer: the evaporator coil determines the size of your distribution ductwork. So, if you up-size the tonnage (and therefore the evaporator coil), you will have to change the ductwork. You may also have to increase fan-size and so forth.

So, the Pro was correct-in-principle even if he did not explain fully. You should have a load calculation done and from that determine the size required and the various upgrades that may also be required (if any).