We have more people with more complex needs. On that note, we need a larger government to meet their needs. We have more people to take care of, and more programs to run. Thus we need a larger government.
I agree that the increase in population and complexity of our world have caused the enlargement of our government. With each change in the world at large, we have to adapt our government to compensate. For example, after 9-11, we created the Office of Homeland Security. This is something that was never thought necessary until the complex nature of war and terrorism changed things. This type of change occurred during the Great Depression as well. People were unable to find work and they needed help. Programs like welfare were created to help people survive this difficult time. The government had to expand in order to protect its people. I don't know that the framers of the Constitution could have foreseen the changes that would take place. We have to take the principles this country was founded on and try to adapt them to work in today complex society.
If you look at the legal system alone (I notice you posted this under "Law") you'll notice how complicated the ideas behind the Constitution have become since its inception. In additioon to the ideas presented above, you might argue that government becomes a little bigger every time a law is broken, brought before a court, re-interpreted, and re-written.
Additionally, with population growth, economic growth, technological growth, growth of personal freedoms, etc. it seems more and more laws just keep being written in response to a so-called socetal need.
In addition to being so much bigger, our country is also much more complicated. When the country was founded, practically everyone was a farmer and life was simple. There was no highway system. There were no TV stations or radio stations or airlines to be monitored and regulated. Nothing that people did could do much to pollute the air (especially) or the water. There was just not that much need for government. Nowadays, there are so many of us doing so many things that you can see (even if you do not accept it) why some would think we need a bigger government.
I would add to the previous post that the extension of our government is necessary based upon the fact that our country is not the same size as it was when the Constitution was created. More people means more work; more wok means more people are needed to run the government. For me, it is that simple.
At the time of the Great Depression, many people (particularly "Keynesian" economists, influenced by the theories of John Maynard Keynes) believed that the only way to stimulate the economy was through massive government spending. New agencies were created, many new bureaucrats were employed, and people were put to work through such programs as the Civilian Conservation Corps. Programs such as Social Security were created so that people would not be totally poor when they became too old to work. The federal government then became even larger during the "Great Society" programs of President Johnson during the 1960s. The governments of many other countries, especially in Europe, also grew during the period from the 1950s onwards. The wisdom of such growth of government has been much debated, but the fact of such growth seems undeniable.