The size of the House of Representatives has been set by law since a statute passed in 1929 limited its size to 435 representatives. The House had reached that number in 1913 and has remained there ever since. The population of the US at that point was less than one-third what it is now. So the point of your question still holds even if your numbers are wrong.
To some extent, this is a problem. It means that people are not represented as directly as they were in years past. If there are three times as many people as there were in 1913, that means that each representative represents three times as many people as they did back then. That means that the voice of every American is not heard as clearly as it was in 1913. Since democracy depends on the voice of the people being heard, that is a problem.
Of course, there is not much we can do about it. A House of Representatives three times the current size would be completely unwieldy and chaotic. Each representative would have even less power than they do now and representation might not really be improved all that much.