Why does a freezer need a bigger suction line than an AC of the same tonnage?

Suction-line sizing balances enough vapor velocity to return oil against acceptable pressure drop. At low evaporator temperatures (≈-20 °F) the refrigerant vapor is much less dense, so a larger suction OD is needed to carry the same capacity without excessive pressure drop. That is why the low-temp table specifies larger suction lines than the AC table at the same tons.