I would argue that the Framers of the Constitution devoted so much of the Constitution to the legislative branch because they wanted it to be the dominant branch of government.  They wanted this because they would have believed that it was the least likely to tyrannize the people.

The Framers were very worried about tyrannical government.  They felt that the British government had been tyrannical and they did not want another such government.  They knew that they had to make their national government stronger than it had been under the Articles of Confederation, but they did not want it to be too strong or too able to act decisively.  One way to achieve this was to give Congress the bulk of the power.

The Framers most certainly did not want the executive branch to dominate the government.  If the president came to dominate, he (they surely were not thinking a woman would ever hold office) would become like a king.  Therefore, they wanted the legislative branch to be the strongest branch.  It included representatives from all of the states and therefore could speak for the whole population.  It had many more members and those members were divided into two houses, both of which had to agree in order to pass any laws.  In other words, A) the legislative branch was more representative of the people than any other branch and B) it was harder to gain consensus within that branch than within either of the other branches.

For these reasons, the Framers thought it was safer to place more power in the hands of the legislative branch than with either of the other branches.  Because they wanted this branch to dominate, they devoted more of the Constitution to it.