I think Wizards choose to do a Gargantuan Orcus (that's assuming Amazon isn't pushing a vaporware product here) for two very simple reasons.
1) The Icons line is all about Gargantuan and Colossal . . . . BIG . . . . figures.
2) The Icons line is all about D&D, well, icons! And you can't get more iconic than Orcus!
We've already got a Large Orcus, and it's cool. We'd probably all love a Huge Orcus in an upcoming set (sculpt depending) . . . . but the only thing that could be cooler than a Gargantuan Orcus would be a Colossal Demon Prince of the Undead!!
Wizards can't produce Gargantuan and Colossal dragons forever, so they are branching out and what better way to start off the experimentation? If Orcus sells well, I see not only more enormous demon lords, but giants, a tarrasque, and other super-big iconic beasties. I'm drooling over the possibilities.
Also, remember, that a creature's size doesn't necessarily correllate with its CR. Just because this new Orcus figure will be Gargantuan doesn't mean it's CR will be in the stratosphere, although it could be . . . .