i'm no expert on this but the whole thing pretty much comes down to the child's attitude and the child's ability to balance ...
btw, here's a pretty good link I found on the subject:
http://bicycling.about.com/od/beginningcycling/a/teachtoride.htmthe elements you should be considering include the child's enjoyment of riding, dealing with fear of falling, learning about bike safety, and learning the mechanics of balancing on the bike. All of these factors are critically important ... it can't just be about the mechanics of riding ...
having said that, I think the best way to learn balance is to learn that "slow" is hard to balance and "fast" is much easier ... if this isn't clear, try sitting on a bike that isn't moving and using your weight to keep it from tipping to the left or the right without touching a foot to the ground ... it's very difficult ... but when the bike is moving forward, it's much easier ...
so, i think the trick to learning balance is to get moving ... with a heavier kid, you need to get moving even faster ... for starters, training wheels are the way to go ... back when i was a kid (back then bikes had square wheels - it was a really long time ago), the training wheels could gradually be raised up higher and higher so that they only "corrected" when i leaned too far over and failed to balance in the middle ... this seems like a good aid to learning balance ...
and then, once the child is comfortable with the training wheels raised up pretty high and is able to ride fairly quickly, it's time for parents to start jogging alongside the bike ... if the child isn't generating the speed, you have to ... the link above talks about holding one hand on the handlebars and one on the child's sweatshirt ... when i learned, my friendly neighborhood jogger just held one hand on the back of my bicycle seat ... again, a heavier child makes it harder but not impossible ... generate the necessary speed and allow some degree of "tip" to the left or right so that the child can make the necessary balance correction on their own ... if they go too far, your strength can keep them from falling ...
the goal is to give them the feel that you are no longer needed to produce the necessary speed or maintain the necessary balance ... it's a question of degree ... in the beginning, you may need to provide most of the speed and most of the balance ... during this time, the child adapts to going faster and starts to develop a sense of balance and confidence ... gradually, you let the child do more of the work to generate the speed and you allow more "tip" to occur before you intervene to prevent a fall ...
if the child is heavier than your capabilities of speed and strength, you might need to find another helper ... sometimes parents reinforce fear where less familiar teachers emphasize meeting the goal ...
hope this helps ... good luck ... let us know how it works out ...