There was an article in the Nisqually Valley News last week (you'll have to take my word for it, its not online) about how the city down there is considering adding two additional spots on their council, increasing the size from 5 to 7. The logic is to get beyond a requirement that would make them expand anyway when the city's population passed 5,000. They are just about there now and increasing now would allow them to arrange elections for the positions instead of having to nominate them by the council.
The logic of the state law that requires cities larger than 5,000 to have larger councils is that the workload for larger cities is better handled by larger councils. A few more representatives allows more deliberation, more representation and better government.
I wonder why the same requirement isn't made of county governments? Since Thurston County was founded in 1852 to today we've had the same number of commissioners representing us.
In 1890 there were less than 10,000 Thurstonians and 3 county commissioners. In 2006 there are over 220,000 Thurstonians and 3 county commissioners.
There was a home rule effort a few years ago to rewrite the county charter that have expanded the number of commissioners or established a council, but that fizzled out.