I don't think the soundness of the device should be your concern here- if you're taking big falls, the GriGri is unlikely to be the point of failure in your system. I'd be more concerned about your top point of protection (which will get 1.6x the force you experience in a fall), your belayer (who should be anchored if you've got a significant weight disparity- she'l get .6x your falling force minimum), or any number of points in your system that are weaker than the grigri. I strongly suspect that if you load your system to the point that the grigri fails, you have other, more serious problems- like, your spine (or that of your belayer, or some other part of your system) will have disintegrated under the load already. Be careful. If you're a lot bigger than your belay partner (I've got 80lbs on my wife, so I'm hip) you've got more to do to protect them- this is easy to learn, and I highly recommend it.

I highly doubt this will break on you. At my local climbing gym we use the gri gri on everywall. Sometimes we get overweight people falling and going up and down all the time. Just trust the gri gri. It is meant for that stuff.

There's no way anyone able to get themselves off the ground is going to be heavy enough to break this thing. And it's not that small. 5-6 inches long. So you should be fine. A bigger concern is that you'll yank your belayer off the ground. Also troubling is the term "toprope lead fall". That's an oxymoron. Be careful.To add to this, remember to use ropes in the proper diameter range of 10mm to 13mm, and you can also anchor your belayer in to the ground so that way there is no concern about your top rope belayer holding you. Gri gri's are almost fool proof, and only malfunction with user error, so be sure you know how to use it safely first!Becareful though, This is not suppose to be given to an unexperienced belay. It has and can get people hurt. Its easy to use but not fool proof. I use grigris alot!!! and I wouldn't say that they ONLY fail with user error, but EXTREMELY rarely. If used properly it is no safer than an atc and MUCH more expensive. It has it's place and does it's job well when used properly, but I have found that it tends to lead to a false sense of security and a an unattentive belayer sometimes. Like I said I use grigris alot in one particular application, but for traditional belay situations i still prefer tube style belay devices. Just like any other belay device when used properly the size of the climber really does not factor into the stopping ability, with the grigri however it CAN affect how smoothly it releases. Also if you do choose the grigri make sure you use at least 10mm or larger rope and keep the device CLEAN, dirt in the wrong place can cause it to stick in the wrong position.