I have heard this. I'm not a military man, and not knowledgable on the subject. But I do have the idea that we may not have a large enough army to handle all the potential threats that today's world poses, that is, if very many of them manifest at once.
If memory serves, however, 19th century Britain conquered the world with tiny armies that it sent to all continents. Sometimes, though, because of this, they got their heads handed to them in a basket. India, for example. But they always sent a new force later to set things right.
I have wondered if there was not something of a parallel here for the US.