I agree. I don't know how big this van der Sloot kid is but I'm guessing that he would have trouble disposing of a body all by his lonesome. They apparently did a pretty good job of it if she hasn't yet been found.
Joran is about 6' 5" and about 200# - He is fairly "buffed" up from some photos I have seen.
Sloot is 6ft 5in. I'd guess 200 pounds. The car is owned by the brothers. Three guys would be able to hold down a 110 pound 5ft 4in girl, easily, rendering her helpless. One holds wrists, another ankles and the other is on top of her.
JVDS: 6'5" - 190+ pounds
NH: 5'3" - 103 pounds
My guess is he'd have no problem at all disposing of her body.