I agree. It's undersized. Is that the only return trunk? If it is, you need to have a serious talk with your installer. Chances are, if he isn't doing the return properly, other stuff may well be done wrong.

BTW, that rating is only for smooth metal ducts. Anything else is lower.

Hart & Cooley is a company manufacturing heating & cooling products such as grilles, registers, diffusers, vent systems.
Dave Fetters from Hart & Cooley says that a true rule of thumb to use is to plan for two cfm for each sq.in. of gross grille area.

For example, a 20-in. x 25-in. grille has 500 square inches of gross area (500 x 2 cfm= 1,000 cfm). Using this approach, two 20-in. x 25-in. return grilles will be the right choice for a five-ton HVAC system in order to actually deliver 2,000 cfm back to the blower without causing the return to hemorrhage.

24x8 is a bit undersized if you ask me. Better to have 24x10 or 20x12. How loud is the system operating with this return?