Just measure the length, size and height of your bag, and compare it against what the airlines allow (the limit of L+B+H, called the linear size, is usually 114cm). But backpacks have more leeway because they are soft, and as long as you don't stuff it full, you can squash the bag to a smaller size when stowing it.
If I want the volume of a backpack without specs, I measure the size, and then take 75% of it. Eg, if LxBxH = 40L, I just take 0.75*40L= 30L.
Those people with big bags are trying their luck. Sometimes, the staff will allow, sometimes they won't. If you try your luck and it's not your lucky day, you'll be forced to check it in, and if you are on a budget airline, you have to pay $20-30 to check in your luggage.
Also, just because the counter staff allow you to bring in your bag doesn't mean the gate staff will allow it. So better to be safe and ensure your bag is not too big.
mine china bag no put liters..
but sometime i saw the angmoh bring very big. how come they can carry in? or is it just pay money will do?