The population tiebreaker is based on State population of the most recent census at the time the Senators took office. So Tester won't overtake Whitehouse, as the 2000 Census results haven't changed. If there's any overtaking, it'll be if Alabama unexpectedly passes Tennessee and they redo the tiebreaker between Tuberville and Haggerty once the 2020 Census results are finally released.
Is this even possible, since the 2020 Census wasn't formally completed by the time they were sworn-in on Jan. 3rd?