I too feel South West policy makes sense.
On LCC's which do not offer children's discounts it seems ridiculous to one one hand charge a high excess baggage rate while simulataneously charging the same rate for a just 2 year old child (max free weight of child plus allowage luggage 40kg) and a 170kg man (190kg with bag).
I certainly remember one flight where a rather 'large' passenger raised the arm rest and basically pushed my small son out of his seat and was offended when the boy (too young to understand) asked me why the man was doing it.