[bitwize]

The current thinking is that T. rex used its stubby arms to grasp prey when it went in for the kill. A similar line of thinking applies to the flapping wings of birds: earlier theropods flapped their forelimbs in order to help hoist themselves on top of their prey to make the killing bite.

Their common ancestors may have evolved bipedalism because forelimbs proved cumbersome when taking down large prey.

[bobsil1]

Mountain lions use front paws broader than rear to grasp large prey as they bite the neck. As large as elk and deer.

[bitwize]

We're dealing with larger than mountain lion scale beasties here. Fun fact about hippos: they can't jump. Like at all, unless they're submerged and can use the buoyancy of the water. The same is true of just about any modern tetrapod within that size range. If you're a tetrapod the size of a mountain lion or even a bengal tiger, you can still have speed, agility, and the ability to spring, pounce, and grapple with the forepaws. At T. rex scales, the square-cube law has something to say about that.