The main difference in the size of spring bars is its relative stiffness. The overall dimensions (the ones that count) will be the same. I have found that going with a rating too much over your tongue weight will make the ride less progressive and twitchy near the limit. I tried this with my old TT to see what the difference was I went back down to the 1,000lb bars after a few trips.
Remember, the bar's main function is leverage weight towards the front truck axle and collaterally the trailer's tandem axles. A stiffer bar will give less and transfer that weight during bumps and dips faster and more harshly than a lighter weight bar.
You can also expect greater wear to the points of connection as more strain is added to them with each bump. Not that it is much more wear. I did notice more metal dust at the end of the few rides I did take with the 1,200lb bars.
All that said, that was just my experience. Other than the cost of the new bars, there shouldn't be any harm in trying them.
