Rosseau believed that government should be designed to govern according to what he called the “general will.”
[Political] institutions should serve the community of citizens and not the most powerful social minority … [and] the government’s mission … is not to amass state power and wealth for itself. (arcade.stanford.edu)
In other words, the government really does exist for the people. The government is supposed to protect the people and not get rich off of them.
Montesquieu believed that any man given power would abuse it, and therefore there needed to be a separation of governments and a system of checks and balances.
This is achieved through the separation of the executive, legislative, and judicial powers of government. If different persons or bodies exercise these powers, then each can check the others if they try to abuse their powers. (plato.stanford.edu)
Thus both men believed that government should protect the people, and that the strongest government is one that exists for the people.
Montesquieu thought that the type of government a society should have was connected to certain realities of the society. Generally speaking, the larger the country, the stronger the ruler needed to be. Smaller polities could be more democratic. Rousseau's focus was more on the ability of government to respond to the will of the people. He favored smaller, less complex government as a general principle because he thought it more responsible to what he called the "general will."