After Mr. Loftus updated his scale charts I noticed the talk of the Harvester being much larger than I realised. So I got to reading up about it, and found this on Halopedia:

"Throughout the Halo 4: Official Game Guide's digital Spartan Ops walkthrough, the Harvester is constantly referred to as being a Forerunner construct."
http://www.halopedia.org/Harvester

I thought this hard to believe, so when I looked at the images Halopedia has for the Harvester I realised I had forgotten how insectoid it was:

http://www.halopedia.org/File:Harvester_Sculpt.jpg

It definitely doesn't seem like your classic Covenant architecture; being in my eyes even more of a departure than the Scarab. In fact, it reminds me of the description of the Timeless One:

The Primordial was massive in size; the mold encapsulating the being on Charum Hakkor outlined a being approximately fifteen meters tall and eleven meters wide. When first witnessed by the Didact on Charum Hakkor, the creature was described as possessing an insectoid head, four arms, and two degenerate legs. A long, segmented "tail" was attached to the base of the skull, tipped with a two meter-long barb. Each hand had three fingers and a central opposable thumb. The face of the Captive has been likened to that of a eurypterid, or "sea scorpion," a creature that the Precursors had allegedly seeded on a number of planets. Its head was flattened, with oval compound eyes and insect-like mouthparts.
Later, aboard Installation 07, the creature was described as having a large number of legs which it held in a curled-up position similar to a spider. The creature's torso was described as "grossly fat," and its skin was covered in a glassy, crystalline coating; a fine powder often fell from its body, perhaps Flood spores or the same powder responsible for the original Flood outbreak. As a Gravemind, the Primordial was capable of shifting its physical form to an extent, such as rearranging its limbs.
http://www.halopedia.org/Primordial#Appearance

Visualised courtesy of Postmortem, TDSpiral and Hedgemony here:

http://forums.bungie.org/halo/archive39.pl?read=1168369

So, could the Harvester in fact be more Precursor than anything else? It definitely doesn't look Forerunner to me, and only Covenant in passing.