The expansion chamber doesn't cause high rpms, it makes more power. Usually at high rpms because there is more power there, HP being torque x rpm, but if there was a reason to make the power peak lower, the pipes can be tuned for that.
At the rpm the pipe is tuned for, the expansion chamber reflects a low pressure wave back to the exhaust port as it opens and then a high pressure wave as it's closing. That pulls mixture though the transfers and even some out into the pipe, and then stuffs it back in right as the port closes. The exhaust can muffler on a saw, if it's the right size, does the same thing, only much less efficiently. Some saw mufflers have multiple chambers or short tubes in them, and that's often to get the right waves at the right time. Sometimes it's to reduce noise.
Because a saw needs to be light and compact, it can't fit a big tuned chamber. Instead they make the engine larger, with a larger bore. That adds only a little bulk and weight. There is no displacement limit on working saws like with racing motorcycles, so there's no need to wring all they can out of a limited displacement.
