Well my guess is you aren't shifting as well as you could, IE, not shifting under load. You want to learn to soft-pedal when you shift so you aren't as stressful on the chain and drivetrain... thats a huge cause of failure and is really exaserbated by a larger rider.
For durable stuff, look into downhill and freeride parts. They aren't cheap but they are strong. Best value in wheels are Sun Ryno Lites on Shimano hubs... you can get a set for under $200 usually. Cassettes, just get a LX or XT cassette and thats about as good as they get. For chains I always buy SRAM, but the Powerlink system I think may be a problem for big riders.