Bell has increased the max gross weight of almost every model they've ever produced, at least in the past 50 years. They come up with a weight, and when the customers complain that there isn't enough payload, they increase it. No reason the 429 can't be increased also, the 412 weight was increased more than once. The real limiting factor is the landing gear, and if they can show the gear can handle the weight, it's easy enough to do. This is the reason the allowed weight for slingloads is higher than for internal loads.