Just for clarification I'm talking about the in-game weights.

This will be talking about how much of a T. rex each creature weighs (i.e. a T. rex weighs 1/1 of a T. rex). Of course, these won't be exact most of the time due to the somewhat bizarre nature of the creature weights, but they are usually within 1% of 1% (aka very close).

1 represents a T. rex's weight, so 1/2 means half of a T. rex while 2 means double a T. rex.

Tyrannosaurus- 1 (duh)

Giganotosaurus- 1+1/6

Spinosaurus- 1+2/3

Triceratops- 1+1/2

Shantungosaurus- 2+1/6

Allosaurus- 1/2

Carnotaurus- 1/3

Really the point of this post though was to show that the numbers are really arbitrary, and that the odd nature of them is essentially just to cover that up. (Edit) They're actually from more exact conversions from pounds to kilos. Still arbitrary, and unecessarily exact though.

The examples above could be simplified to the following:

Tyrannosaurus- 5,400

Giganotossaurus- 6,300

Spinosaurus- 9,000

Tricerratops- 8,100

Shantungosaurus- 11,700

Allosaurus- 2,700

Carnotaurus- 1,800