I'm going to show what the effects would've been by decade. Here are my assumptions:
If a state that was below the threshold IOTL when it was admitted ended up meeting the threshold with in the next 2 censuses, I'm going to assume congress would've been patient and waited. For instance, Illinois was too small when it was admitted in 1818, but by the 1830 census was sufficiently large and would've been let in.
If the state did not meet the threshold within the next two censuses, I assume that congress would graft it on to another state or territory to provide the people with representation. Florida for instance, did not meet the threshold in 1845 and was still below in 1860 and didn't cross over until 1930, so it gets joined with Georgia.
The one exception I'll make is Mississippi. It and Alabama were once a single territory and I don't think it makes sense to split them up even though Mississippi reached the threshold in 1830.
1790-1810: All states meet the threshold, no effect
1820: Illinois is below the threshold. Mississippi is below the threshold and the territory is admitted without Alabama being split off
1830: Illinois now meets the threshold.
1840: Arkansas does not meet the threshold.
1850: This is when the rule starts having real effects. California, Texas, Florida, and Iowa are below the threshold and have to stay out. Florida, being a humid, misquito infested swamp where few would want to live, is combined with Georgia.
1860: Texas, Arkansas, California, and Iowa now meet the threshold. Minnesota and Oregon are too small. I imagine that the territories of Texas and Arkansas see confederate coups and join the civil war just like IOTL. Both territories may be forced to wait until Grover Cleveland's presidency for statehood.
1870: The threshold does not prevent any confederate states from rejoining. Even West Virginia is large enough. Minnesota now meets the threshold. Nevada, Kansas, and Nebraska do not.
1880: Kansas and Nebraska now meet the threshold. Colorado does not.
1890: This is when the effect of the rule is at its highest. The Dakotas, Montana, and Wyoming do not meet the threshold. They are merged into one state. Washington and Idaho do not meet the threshold, they are merged with Oregon and become one state. Utah does not meet the threshold.
1900: Colorado still does not meet the threshold. It is merged with Utah, Nevada, New Mexico, and Arizona.
1910-1960: No effects
1960: Hawaii and Alaska are too small and forced to remain territories.
Today: Only Puerto Rico meets the threshold and as its population shrinks, it probably will fall below.
So, the US has 37 states now. The list of smallest states is now dominated by the Northeast instead of the Rocky Mountain west, with Vermont, Delaware, Rhode Island, New Hampshire and Maine, in that order, being the smallest. Georgia, with Florida grafted on, now has 32 million people, making it larger than any state besides California.