Why are intake ports on low- and medium- temperature compressors larger in size than intahe ports on air conditioning compressors?

To begin with the suction line is always larger than the discharge so it can facilitate the movement of the required volume of vapor, which because it is at a lower pressure is less dense so it needs more area (larger pipe). In a low or medium temp refrigeration system the suction pressure is lower still, so the gas is even less dense, thus even more area (larger pipe) is required. Additionally, as the design operating temp of the system is lowered so is the rated tonnage. Therefore a 20 ton freezer system will be physically larger than a 20 ton AC system.