The larger opening on cans, while a homologous structure to the mouth of certain lake-based fish, serves a single purpose: to entice the customer into consuming more product.
What you say!!
A larger aperture to the can allows for quicker dispersal into the mouth. The faster distribution of fluid allows for less air to be consumed along with the beverage. This means the drinker's stomach won't fill up as quickly as it would with a "normal" sized opening. With the old way, one would release a small amount of the trapped air in a burp, but still carry around a large quantity, making one's stomach feel "full." At this point, one would stop consuming the beverage.
By enlarging the mouth and permitting freer passage of drink, the soda/pop/beer/Perrier manufacturers make you feel you need to drink more to achieve a "full" state, and therefore they make more money -- which is ultimately what any company wants to do. | 
