Ideally you'd see from the get-go is a "Manual-J" heat & cooling load calculation, and a Manual-D compliant duct design for supporting those loads. Most commercially available load calculators such as Wrightsoft use Manual-J methods. But that's almost never going to happen for just extending a zone. If they can't come up with numbers for at least a HEATING load, they're just taking a wild stab, probably oversizing the ducts "just to be sure", which won't deliver stable/comfortable basement temperatures.
Being largely below-grade, basements have very different heat loss characteristics than above grade zones, and often little or no sensible-cooling load coupled with a higher latent load from groundwater moisture diffusing in through the foundation and slab. If it's a fairly open space you may be better off with a 3/4 ton mini-split making the basement it's own zone, rather than marrying it to the first floor zone, or adding it as a separate zone for the main heat pump.
You can do your own I=B=R methods for calculating the heat load, if you know the U-factors of the windows & doors, and the construction/insulation stackups.