That's because it's mostly wing. It dates from the Dash-80 era of slim, pure jets and low-bypass engines. Today's high-bypass engines are big shrouded fans with a jet engine at the center to spin them. Fuel economy is much better, but the engines look bloated.
It's just a function of when the aircraft was designed. The WC-135 aircraft are modified C-135s produced in the 60's. The C-135 was designed in the mid-50's. At the time, high-bypass engines simply didn't exist yet.
