Extrapolating from this data the current reality means Austin is probably larger than Columbus and similarly sized as Cleveland and Kansas City and, if not yet, marginally larger than them then within the next year. I'd say we pass them at some point in 2017, Cincinnati in 2018, and then Pittsburgh in 2020. Then, beyond them Austin would then have to surpass Vegas, Sacramento and the cities discussed below. So, after surpassing all of those cities, it'll be until the mid-2020s that Austin moves up further, except sporadically (Portland by 2022 or 2023, maybe), but by that point who knows (a) what cities will still be growing, (b) whose growth will have plateaued (will ours?), (c) and how that will affect rankings shifts. So, I really only think in terms of the next 10 years (out until 2027ish), at which point we'll still be behind any of the next larger cities (San Antonio, Orlando, Charlotte) are all growing pretty fast as well.
San Antonio and Orlando are probably already larger than Portland (Orlando is growing faster than San Antonio), but then there's gonna be awhile before they move up in the rankings again because Charlotte is also growing fast (that's the next city up) and then there's a large population gap between them and Baltimore/St. Louis/Denver/Tampa (two of which are still growing, if slower, and two of which are growing fast).
And then do we have to think... what cities could possibly outgrow San Antonio or Austin during this time period? Probably nothing. They have two of the fastest growing populations in the country. Perhaps Vegas could outgrow San Antonio by the mid 2020s, maybe, if they had another sustained period of exceptional growth for a few years and then built off of that for awhile... but nothing is gonna outgrow Austin at all. We're the fastest growing metro over 1 million five years running.
http://www.statesman.com/news/news/local/williamson-county-ranked-seventh-fastest-growing-i/nqrcG/