Principle-agent problems between union leaders and members aside, unions want to obtain "more" from their employers. They are best-able to obtain more from employers when their employers have market power in their output market. Having market power allows firms to earn economic profits which the union can then attempt to capture. So, as the always-astute Coyote observes, it would be no surprise to find out that unions are behind the San Diego city council ban on Wal-Mart Super Centers.
A Wal-Mart Super Center differs from a large Wal-Mart mainly in that is sells groceries. And in fact the legislation does not ban all super-large stores, just ones that sell groceries (Wal-Mart could still build a super-honkin large store in San Diego as long as it didn't sell food). Well, this should give us a clue. It tells us that the politicos are not against large stores, just against large new stores that compete with existing grocery stores.
And this puts the lie to supporters statements that their concern is that Wal-Mart "puts smaller competitors out of business." There aren't any "smaller competitors" in California grocery stores, they are all large chains run by corporations. And if there are any local fresh produce shops out there, I don't think their customer base is one to run off to Wal-Mart. This is about protecting grocery retailers from competition. Why? Well, there is one other thing we need to know, and that is that California grocers all have extremely powerful and politically connected unions.
Unions are organizations that serve workers, not the general populace or workers in general. Union leaders that use scarce union resources to obtain things for others at the expense of the workers they serve are not going to be union leaders for very long. It's only by coincidence that the interests of the populace in general and unions align.