Every bike is different, but I think you've pointed it out already. The bike is designed for a average 180 pound rider, and not one sitting forward either. That will effect the rear braking. It should work better with a heavier rider, or with the addition of a passenger. You can check the rear rotor and make sure it's got a good surface on it. As in not rusty or stuff on it. Like rust, wax, grease, chain lube, etc. Maybe clean it with something proper and see if it helps. The R6 service manual does not say what or how to clean the disc rotor. The owner's manual, however, says to "...clean the brake discs and brake linings with a regular brake disc cleaner or acetone,...". Do not soak your skin with acetone or brake cleaner, nor breath acetone fumes. Use it with adequate ventilation. There is a thing where gunk builds up behind the piston, and it will drag, or slowly lock the piston to the rotor. But I wouldn't expect it on a newer bike. There aren't many reasons why a rear would lock easy, I guess I'm a bit stumped. I've seen it with drum rear brakes, but not disc. I'd rather not get into the front or rear or both brake argument. It's a brake, learn to use it for what it's worth. And it's worth a lot, or it wouldn't be there.