Brutus (the Anti-Federalist writer, not Shakespeare’s character) believed a republic as large as America would be dangerous because it would be too big and too diverse for truly accountable, representative government.

Core reason in one line

He thought that in such a vast republic, ordinary people would lose real control over distant rulers, letting power concentrate and slide toward tyranny.

Brutus’s main concerns

  1. Distance and size break the link between people and rulers
    • The United States covered a “vast extent” of territory, with millions of people even in the 1780s.
 * Brutus argued citizens would “become acquainted with very few of their rulers,” so they would not know them personally or trust them.

 * If people don’t really know their leaders, they can’t watch them closely or hold them accountable.
  1. Too few representatives for too many people
    • Under the proposed Constitution, only a small number of representatives would speak for a huge, growing population.
 * Brutus believed this meant those representatives could not truly “speak the sentiments” of such a large, varied public.

 * With so few in power, it becomes easier for them to form an elite group and serve their own interests rather than the people’s.
  1. Diverse interests make common good harder
    • A continental republic would include many different economies, customs, religions, and local interests.
 * Brutus thought a single central legislature would be pulled in many directions and constantly divided by “heterogeneous and discordant principles.”

 * In that situation, he warned, the “public good” would often be sacrificed to local factions and special interests.
  1. Risk of power concentrating into tyranny
    • Because the national government would be strong and distant, Brutus feared its officials would “gratify their own interest and ambition.”
 * In a very large republic, it is “scarcely possible” to call such rulers to account or stop them from abusing power.

 * For him, that made a large republic unstable as a “free republic” over time; it would tend to drift toward oppression.

How he imagines a better alternative

  • Brutus favored smaller, more local republics (like the individual states) where:
    • Representatives live closer to the people.
    • Citizens know their leaders personally.
    • Diverse interests are easier to manage on a local scale.

In short, Brutus believes a republic as large as America would be a problem because its great size and diversity would weaken representation, disconnect people from their rulers, and allow power to concentrate in a distant central government, increasing the danger of tyranny.

TL;DR:
He thinks the country is too big and varied for one central republic to stay truly representative and free, so over time the national government will become unaccountable and oppressive.

Information gathered from public forums or data available on the internet and portrayed here.