A larger U.S. population means more work for Congress. 

And we need to avoid concentrating power in

too few hands.

Frozen Apportionment: 

More Issues

The floor ratio the Constitution establishes, no more than one Representative for 30,000 people, keeps their number small enough that no one can have their own personal Representative or their own family Representative. 

But freezing keeps the House too small. The number of Representatives remains 435, based on the 1910 population of 92.2 million, in 2014, when the population is 316.1 million.
This concentrates power in too few hands, and prevents close contact between constituents and their Representative.And Representatives at large change the way the Constitution designed one House of Congress, creating, to that extent, two Senates instead of one Senate and one House.

All limits on the size of Congress limit the work Congress can accomplish. A larger U.S. population means more work for Congress. Let's not make their membership too small for us. Some have claimed that a too-large House would be inefficient, reducing the amount of work they can accomplish. That’s not quite accurate. Once you have a system to coordinate 100 people it’ll work about as well for 1000. 10,000 isn’t as easy, but a system can be made. Two-factor districting (see the Leashing the Gerrymander pages)  controls for state size far better. It’s foolish to claim a larger United States population doesn’t require more work. Too few members is a far worse problem than too many.  

And allocating Representatives by population gives people power versus corporate interests. States' economic power varies by need (more population = more need) and also by trade (corporations = more trade). But corporate power doesn't have to be shared with employees. By holding down the number of Representatives, international trade generates corporate power in Congress that doesn't have to be shared with the population either. Having Representatives increase with population helps balance corporate power versus the population.   

535 is a tiny body to legislate for a population of over 300,000,000. The entire Senate could convene in a fast-food restaurant. The House is meant to grow with population.