Well, consider this:
You, a medium-sized human are grappling Fluffy, a tiny-sized cat.
Step 1: Grab fluffy with a touch attack
Step 2: She tries to claw your face off to get you to release her.
You win; you are now grappling fluffy.
End of Turn
Fluffy's turn:
Fluffy has the following options:
1. Try to escape
2. Try to claw your face off
If Fluffy tries to escape, she's making either an escape artist check to wriggle free of your grip or a grapple check to claw your hands up until you release her.
If Fluffy tries to claw your face off she's effectively making a grapple check to get to and claw off your face. That sounds like climbing on an opponent.
EoT
Your turn.
You want to climb on top of Fluffy and pound her face in. Sounds like a (really easy) grapple check.
Then you want to grab her by the scruff of the neck so that you can throw her, like a weapon, at her owner who is really fucking pissed at you now. Getting her by the scruff of the neck -- grapple check at -X (where X is large) because she's trying to claw your hand off and you're aiming for a not-easy-to-grab target when in grapple with a cat. Then you have her contained and you're one-handing her -- this is improved grab! Then you throw her at Sally, her increasingly pissed owner. Fluffy does damage as a 10-lb. object made of flailing claws; something like 2d4.
EoT.
Later: Fluffy has plotted her revenge and initiates grapple with your foot. Unbeknownst to you, Dr. Utonium has spilled some Chemical X into Fluffy's water dish and she now has a strength score of 56. Oh, shit.
Fluffy grabs your foot and rears up, lifting you off the ground bodily. (Improved Grapple check!)
Invisible GrappleShe then jumps, does a flip in the air and slams you to the ground. (Stop, drop and roll!)
You are now Prone, crushed under the powerful throw of a measly cat. (And probably dazed and confused by being bodyslammed by a cat.)
So what did this take for a cat to turn into Mighty Mouse? Well, assuming you have a strength of 10 and BAB 0 for a grapple mod of +0. The cat, being tiny, has a size mod of -8 and strength of 56 for strength mod of +23 for a net grapple mod of +15.
If stop drop and roll imposes a -15 to your grapple check but leaves the opponent prone in the square and both of you out of the grapple then a cat needs a 56 strength (!) to have a 50-50 chance of doing that to you!
I think the -8 net was pretty significant there and required a ridiculous cat to hit it.