It is not because if their weight. It is because they occupy TWO seats, and the arilines make money on seat miles.
SW is not the first to do this; other airlines have done this for YEARS.
You take two seat, you pay for two seats.
This is really very simple. Use the SWA test (Sooo Wide Ass) test.. Do the SWA "seat test" before boarding. If ya don't fit into the seat...ya buy two tickets, lift up the armrest and be comfortable.. Maybe they'll give a discount or a "buy one, get one free"...:~)
sw