There is no set amount of square footage per gallon a mopping bucket will cover. Often, the determining factor is how dirty the water gets before needing changed.
There are several reasons why it's hard to estimate how much space can be cleaned before needing to refill your bucket with clean water. The larger the cleaning mop bucket, the more warm, soapy water there is to dilute the dirt and grime. However, a routine, low-risk cleaning will get more use out of water than a high-risk, or heavily soiled mess or spill. This is because you'll want to empty the bucket much sooner to avoid cross-contamination while cleaning up heavily soiled messes.
For smaller establishments, using too large of a bucket can cause unnecessary water waste. Plus, more water requires more cleaning solution, which is costly and inefficient. With smaller establishments, there is likely less distance needed to travel between water sources, which puts much less burden on the user when a water change is necessary.
Larger establishments typically see more day-to-day foot traffic, which allows for more outside contaminants or messes to occur. With larger area to cover, a higher-capacity bucket could allow the user to travel a longer distance before needing a water change. This, however, is still dependent on the type of cleaning being done and what type of mess is being cleaned up.