To put it simply, one million dollars weighs 10 kilograms. Ocean's 11 were stealing 150 million aproximately so that adds up to 1500 kilograms or a ton and a half of money!
How did they carry it outside the casino?
After Benedict realizes that the whole robbery was pre-staged and that the SWAT team he called were actually the robbers, the camera shows them walking out of the casino in their SWAT gear, each carrying large bags, supposedly full of SWAT gear, but actually full of money.
This also closes the circle to the earlier gathering scene where Danny and Rusty explain the heist to the rest of the team:
Saul: Yeah, well, say we do all that. We're just supposed to walk out of there with $150 million without getting stopped?
As to how they managed to carry all that money, let's look at the details. The fake SWAT team was comprised of at least 7 people, maybe even more on the way out, since the initial 7 would be joined by Linus and Yen (which leaves Virgil at the car remote and Danny in supposed confinement). And while Rusty only carries one bag, the guys behind him seem to each carry two of them. This makes about 7-9 people each carrying 1 or 2 bags full of ~90-150kg of money.
Yes, this might be quite a stretch even for a well-trained human being. However, this is way inside the bounds of pluasibility that the movie's setting establishes, given that noone really pays attention to how much money actually weighs. If you're just worrying about the specificities of how much money they could really carry out in those bags and if 50 million would have been more plausible, then I'm afraid you might be the only one among the audience and the filmmakers worrying about that. ;-)