In reality, the "giant" spider was truly tiny. This was a case of forced perspective coming into play, wherein the spider's location relative to the camera made him look much bigger than he actually is.
"Who needs Hollywood special effects?" the police department wrote online. "This little guy was positioned perfectly on the windshield of the patrol car and gave our clerk, who was reviewing the video, quite the 'Halloween scare.'"
Take a bow, little spider!