Yes, a 900 lb. carcass is too big. They get docked not neccesarily at the slaughterhouse, but on the rail they would. The Grand carcass steer's hanging weight was 702 lbs.
In June when we got the figures for our carcass contest, Packers in our area were docking for carcasses over 950 which is roughly equivalent to a 1500 pound live animal!! I think some may dock for anything over 900 pounds.
Not all slaughter plants are the same. some have a 900# cut off yet the one near my house has 1000# cut off.
A 1500# live weight is roughly 940 carcass weight..... dress percent at 62.5%