"Witnesses describe the drones as being as loud as lawnmowers, with some approaching the size of a small car – significantly larger than a typical quadcopter or multirotor drone that anyone can purchase. “These are not necessarily just small, hobbyist unmanned aerial systems that you can buy for $2000,” says Daniel Gerstein at the RAND Corporation, a think tank in California. “These feel like they have longer range and are more sophisticated than what you can get at a hobby shop.”
https://www.newscientist.com/article/2460438-what-are-the-mystery-drones-flying-over-the-us/
"Drone" simply means unmanned, it has nothing to do with the size. The US has some quite large drones in its fleet that are almost as big as fighter jets. There is a reason to be concerned.