Undertaker has tombstoned Vader before, and at one point held Vader with just one arm. Vader is announced as 458 lb., while Ross has said in his Ross Report that Big Show is 414 lb now.
Tombstoning someone isn't a heck of a lot different than powerbombing him. With a powerbomb the guy jumps and it'd be UT's job to hold him on his shoulders, and I think holding someone on your shoulders is easier than holding him with your hands.
And with the move in question, Taker did "hold" Big Show for about a second or so, so a "real" last ride probably isn't too far out of place..assuming Big Show can continue to lose weight and improve his vertical.