In co-flowing microfluidic device, it is usually thought the droplet size is significantly influenced by the capillary number in dripping flow. However, it was found in this study that when the contact angle of the dispersed fluid with the dispersed fluid channel was sufficiently large, the droplet size would be little affected by the capillary number. To explain this phenomenon, an "interface - shrinkage - driven" breakup mechanism was proposed. Three different systems and six different microfluidic devices were used in the experiment to prove the universality of the mechanism. The critical contact angle of the dispersed fluid with the dispersed fluid channel was finally discussed.