You're severely underestimating the Ecumene. THeit total planetary assets included millions of habitable worlds and artificial stations, billions of warships (during one of their final assaults a single Forerunner commander yielded 700,000 warships) including countless scores of massive Fortress-class vessels. Their larger vessels are the size of small moons, and their largest constructs range from Jovian sized structures to solid spheres the width of Earth's orbit.