[@MelonHead]

Who says it's the back that it's hanging over? I believe it would be mostly crouching over the front of the mech, looming over the optics since it not only slithered that way to escape the grab, but also was partially pulled by the mech itself. It's in a different position than when it used the rock. There's also a difference between laying down and sitting. When it is laying down, its main body is eight feet. When it sits, it doesn't take up too much more than half that, as long as it keeps its tail out of the way. If it were fully laying down on the mech's head, it wouldn't have the height needed to swing down its heaviest blows on the optics. 

Also, two more things. In your post you said that it "flung itself back" to escape Higan's sight, however it never moved away from where it had been pulled. Also, its slime should too up what few spots of vision Higan has left.