I don't know how it works exactly but assume it's in the city's zoning code. Thinking more about it, there's not a single superstore in the city. A Costco and several Targets are immediately outside of city limits. It's not even that small of a city--around 100k pop.

There is a Walmart Supercenter close to where West Palmetto Park crosses US 441. I don't know it that is outside the city limits.

Yes, the city limits are approximately at Powerline, with deviations here and there. Area to the west is unincorporated.