Revealing mistake: In the scene, when the bug (actually another Kothoga monster) run out from the box that Margo has opened due to strange noises, there are visible wheels from the puppet of the bug. (00:48:40)
Revealing mistake: When the Kothoga grabs the one SWAT member and starts to rip his head off, it's obvious that the guy is really CGI.





Answer: While the size is never explicitly stated, the creature does seem to be somewhat larger than a tiger and approaching the size of a horse when the scene needs it to be. Perhaps its size changes as it eats and needs sustenance?
Erik M.