Dab rigs are smaller in size compared to traditional flower bongs. Dab rigs put a heavier emphasis on air-flow whereas flower bongs focus more on percolation. Smoke from dry herb combustion is harsher and contains more potentially harmful particles than concentrated vapor. Which is why flower bongs place such an emphasis on percolators and size. Combustion doesn't happen when vaporizing concentrates, and while water filtration is important for cooling and smoothness, too much water filtration isn't ideal. Too much water filtration or heat in a dab rig can lessen the terpenes (the stuff that gives you the "high" feeling) and flavor. With oil rigs you want the vapor to pass through the rig as quickly possible so it doesn't get stuck to the walls, hence such an emphasis on air-flow and compact size.