Now even though they refer to 'all', as a plus-size brand they're really referring to plus-size women, which is clear from the images they use in their campaign. Plus-size women are, of course, typically overweight/obese, as defined by a BMI exceeding 25.
What Lane Bryant don't mention is the unfortunate reality that being overweight/obese significantly increases the risk for a large number of very serious diseases, including heart disease, diabetes, cancer and osteoarthritis. In the United States alone, obesity is estimated to cause up to 365,000 deaths per year, while 1 million annual deaths in Europe are attributed to excess weight. On average, obesity reduces life expectancy by six to seven years.
So Lane Bryant, if you're saying that a medical diagnosis of heart disease or diabetes and premature death caused by being overweight/obese is sexy, you're deluded! If they really cared about their customers rather than making money, they'd instead be promoting the idea that for the overweight/obese, losing weight is the healthy option.
So we now have the daft situation that sexy is essentially being promoted by clothing brands in terms of being either underweight (Victoria's Secret etc.) or overweight (Lane Bryant, Curvy Kate), both of which are very unhealthy! Is there any brand willing to say what most people actually think, which is that what's sexy is normal weight?