My 0.02€:
They stop those big attacks because they ran out of beings to spare*. While they weren't the best of fights, we held them at bay and they took casualties (grievious ones, I assume) so they realised head-on won't work, especially since they have even less numbers now. So now, they start doing smaller operations in an effort to attain their war goals without attracting the full attention from everybody.
Also, IIRC they just about got what they wanted from those battles. No need to kill everybody again for that.
I sure as fuck hope these things aren't soldiers. It'd make Humanity the baddest motherfuckers in the galaxy by comparison.
Who knows, maybe we are. They just have an absolutely illegally huge amounts of guys ready in reserve.
*Sure they have more somewhere, but are you sure they can just warp in more? Maybe their FTL limits the mounts of ships they can put here and especially at the start of the campaign they are seriously lacking in usable bodies.
PS. Welcome to the forums, Vassilios de Veritas!