We stopped expanding the House in the 1920s, decided the Supreme Court should freeze in the 1930s, added the last states in the 1950s, amended the Constitution only once in the last fifty years.
The USA and its government continued to grow, but almost all that growth came in the executive branch.
i really think the only reason we haven't created new states — which was a common political tactic in the 19th century, see nevada and the dakotas — is that 50 is a round number