New census figures show St. Louis urban sprawl is growing rapidly, but nowhere faster than the city of Wentzville.  Wentzville ended the 20th century with 7,000 people.  The census bureau says the population is now almost 21,000.  But other cities in the collar-areas of St. Louis and St. Louis County also are growing quickly. O’Fallon’s population has increased 57% in the same period.  O’Fallon has grown by more people than live in Wentzville now; 26,000. Troy, in Lincoln County to the north, increased by 63 percent and now has 11-thousand people.