Your right on the govt attorneys and engineers. But have you ever seen the immense size of the Dept of Education or the Dept of Ag or the Dept of Transportation buildings in DC? They are some of the most massive buildings in the city and you can bet every office is full. What they hell do we need more and more taxpayer funded doo gooders telling the states how to educate their kids and how to grow their crops?
There are several complexes in DC built BEFORE AIR CONDITIONING. They have high ceilings, large windows, and all offices open along large, high hallways.
The USPS headquarters at L'Enfant Plaza has more employees in it than the 4 story Ag building on Independence ~ and that has to do with the impact air conditioning has on space requirements.
The greater number of federal workers in the Department of Agriculture work in field offices all over the country ~ mostly in small towns and rural areas. That's why you have no idea how many there really are.
Given that agriculture is a major American business employing millions of people (directly, or in food processing, sales, services, transport, etc.) the number of folks employed in USDA is, in fact, NOT excessive.