The population estimate from 2016 of the US Census Bureau for the West Coast (California, Oregon, Washington) is approximately 39+4+7 = 50 million. The population estimate from 2016 for the East Coast (Maine, New Hampshire, Massachusetts, Rhode Island, Connecticut, New York, New Jersey, Pennsylvania, Delaware, Maryland, District of Columbia, Virginia, North Carolina, South Carolina, Georgia, Florida) is approximately 1+1+6+1+3+20+9+12+1+6+1+8+10+5+10+20 = 114 million people (same source).
Historically, the level of agricultural technology and associated infrastructure required to sustain massive populations arrived later to the West Coast of the United States than the East Coast, so one might explain part of the disparity in population size as being a historical artifact which will decrease over time. Indeed, the center of population has historically been drifting westwards.
How far can we expect this trend, of the relative proportion of the population living on the West Coast versus the East Coast to increase, to continue into the future?
Question: Are there fundamental demographic constraints imposed by the ecology, climate, and geography of the two regions which prevent one, at "maximum capacity", from having a larger population than the other? Why is one more heavily populated than the other in the present, and can we realistically expect this reason to continue to hold in the future?
The climate of the West Coast is primarily oceanic and mediterranean. The climate of the East Coast is primarily continental, temperate, and humid subtropical. Do these climates limit the number of people that can settle and live there?
For example, Australia has a similar size as the continental United States, but a much smaller population, since most of the landmass is desert which is impossible (with current technology) to densely populate.
As another example, the state of California alone has more land area than Japan, but only about a third of the population of Japan (whose population in turn is greater than that of the present East Coast of the United States). Is this a historical accident, or is Japan's land intrinsically capable of supporting more people than either American coast?
Attempt: Italy, like California, mostly has a mediterranean climate, and has been settled continuously for a long time and appears to have a stabilizing/decreasing population, so makes for a good estimate of a "maximally" settled mediterranean climate area. Its population density is approximately 200 inhabitants per square kilometer. Meanwhile, the current population density of California is approximately 100 inhabitants per square kilometer, with a population of approximately 40 million, so assuming it becomes approximately "maximally" settled in the future, like in Italy, we can assume a "maximum" population of 80 million people for California alone.
France has also been continuously settled for a long period of time and has well-developed technology, so we can use it as a model for a "maximally settled" oceanic climate (since most of France is oceanic). Its population density is approximately 120 people per square kilometer. Meanwhile, the current population density of Washington is approximately 40 people per square kilometer, so we can approximately triple its population to get its "maximally settled" population of 21 million. Meanwhile the population density of Oregon is approximately 15 people per square kilometer, so we can multiply its current population by 8 to get an approximate figure for its "maximally settled" population of 24 million.
So, in total, using population settlement patterns of countries with similar climates in Europe as a model, we can approximate the "maximally settled" population of the West Coast as 80+21+24 = 125 million people.