The Grey Alien trope almost always features a disproportionately large head, which often comes in unusual shapes to boot. There's nothing wrong with the trope, large brains are cool but what baffles me is how their necks don't snap and how they don't get back pain (at least none that we've seen them complain about). Logically they would have to be properly adapted to having larger heads. Human babies for example can carry a head one third the size of their body. But some aliens have much larger heads that can be about twice or thrice the size of a human head.
What is a realistic spinal structure for a large headed alien?
The average human head weighs about 10 to 11 pounds so a grey's head would approximately 20 to 30 pounds in total. With this in mind I strongly doubt that they would look anything like your typical sci-fi alien. For extra precision about the requirements: 1. they must be bipedal 2. no cybernetic enhancements.