I can't imagine any heavy person I know hassling a thin person just for buying plus sizes because people buy clothes for. other people? Unless LW was making a huge fatphobic production in the store about how they were going to cut down this ugly thing to make something that would fit real people. Which is also possible.)

I suspect this is more how the situation went. There *are* people of whatever size who will walk up to a stranger and scold them but I think LW put more into the interaction than she cites.