In addition to the large Spittle Runner on Dirt, there are several large Spittle Runners in the episode Door to Door. They are seen parked on the beach in the holographic simulation of the Earth being destroy by the Armada. Several tiny Irkens are seen on the beach, giving an approximate size similar to the ship seen on Dirt.
This conflicts with the much smaller ship seen in The Nightmare Begins where Zim crushes it with a Frontline BattleMech with Irkens running nearby for scale. This smaller size agrees with Tak's ship, assuming it is a modified Spittle Runner.
This suggests that there are at least two models of Spittle Runner that look identical in every way except their size. A small one Irken fighter and a much larger multi-Irken ship. Soapy Waffles (talk) 20:50, September 11, 2016 (UTC)