At sea level atmospheric pressure us about 14.7 PSI. It's less at higher elevations. That's what is pushing the oil up the pickup tube to the oil pump. Thus, the larger the pickup tube the less restriction to flow and therefore more potential flow. The larger size sounds obvious, but at only 14 PSI any increase is size is going to help. The pressure differential, as mentioned by nm9stheham, is what makes it work. The pump creates the low pressure area.
