Just some additional thoughts.....

Another way to look at the weight penalty vs additional seats affecting CASM is to look at percentage of weight gain vs percentage of ASM increase.

Even a 737-300 is generally flying around weighing between 100,000 and 130,000 lbs. The 737-400 and A320-series are heavier, around 10,000 lb for the Airbii. So adding a few hundred pounds of seats adds only a fraction of 1% to weight.

Adding 10 seats on an A320, however, adds about 7-8% to the ASM's produced, at a very small cost penalty. So CASM on those planes would drop, bring down the overall CASM. While the extra seats would produce extra revenue (in theory) only when sold, that CASM reduction is there the entire time these planes fly.

Which is why I put CASM reduction as #1 on my personal list of "What on earth are they thinking?"

Jim