The oversized barb (5/8" OD) is specially designed for 1/2" silicone tubing. The larger size does two things:
1) It stretches the tubing over the barb creating a tight fit requiring no hose clamps.
2) The inside diameter of our barb is 1/2" compared to standard 1/2" barbs which have a smaller 3/8" size. This creates a full port design and will not restrict flow going through the barb.