Steve, you brought up almost my question. "imagine, if you will, a halfling trying to Grab and restrain a giant", on page 384 of the 5ER this table shows Grab = grab 2 limbs, can squeeze or throw, and on 387 under The Effects of Grab = Grab immobilizes tow the the targes limbs. This seem to indicate that this is the reason a smaller being cannot grab a larger creature, because he is too small to restrain 2 limbs of the other. It would also seem to indicate that the halfling would not be able to Grab and throw the giant. I can see and agree with this in general.
What if said halfling had 10 flight and 50 STR and the giant weighing 6,500. Could the halfling fly and "grab"/lift the giant and fly into the air with him?
Could the halfling fly into the air if the Giant had Grabbed his legs, when the halflings flight was not purchased with the restrainable limitation?