Why did some people want all states to have the same number of votes in the federal legislature? what did they fear?

Because they believed that this system did not represent the voters justly. For example, 50,000 people have represented for one state, but another state would have one delegate for 300,000 people. They thought that the larger state should have had 6 agents, representatives or the delegates so that every 50,000 people have equal representation. But contrary, the smaller states considered they would have had less authority or the power in a federal government if they only had one delegate. So this is the reason that the Senate was organized so that every state would have equal representation despite population.

The people of large states thought that they should have more delegations in the Congress, while small states desired the equal number of representation with the larger ones. Each of the states would be fairly represented in the Senate, with the two delegates, while commission or organization in the House of Representatives would be based upon population.