As said before, Portal 2 was originally going to be set way back when cave was alive, while the project was still going as f-stop. As such, I believe it would involve some kind of photography gameplay, where changing the f-stop (aperture opening), it would alter the world in some way, for example, darkening the scene to eliminate a bright obstacle, or changing the DOF so that you can pass through an obstacle in some way.
Valve have said little about it as they might incorporate it into a future game, and I'd be more than happy for it to be a prequel to portal without portals, and tbh, if they dont do it, they should at least release a tech demo they made playtesters try, so we can see what it was all about and what we're missing out on...
also this is really cool, I forget what this is, is it that psuedo form game (i think it changed names since)
I want. So bad.
But f-stop was sans portals. So I very much doubt it was like that. Though they totally should do that. You can change the size of all portals using commands, but to have different sized linked portals would be awesome, though I doubt the source engine could handle it as well as the engine in that gif you linked is, I believe that was built FOR that purpose. but who knows, maybe source could do it...
Thing is with those portals, going through one would also change the players size. You could be TINY and run through little gaps and then grow massive to bound over buildings. It's a game mechanic I have wanted to use for so long, ever since I saw lionhead's demo of 'the room', that they said they may release as a tech demo and never did SO DISSAPPOINTED =[