Appearance self-esteem takes a hit when they don't fit in a size. They take it out on the clothes: "I hate their stuff, they suck." They buy more of other stuff to compensate for the hit, whether non-sized accessories (I am pretty) or book/tech (I am smart even if I don't fit).
People confident in their appearance are immune to the effect, and simply think it's sized wrong or runs small.
Having read the article, I can't venture a guess without feeling condescending...non-conforming? Compensatory?
Legitimately confused.
Of course, the trick (once you know) is you don't need a comment summarizing it for you. The abstract is alright in a pinch, but the "General Discussion" in psychology papers is the equivalent of "Conclusion" and aims to discuss the results directly. It's still a bit verbose... but the language should at least be very familiar in comparison.
Indirect phrasing e.g., "they respond more favorably to products that can help to repair their damaged appearance self-esteem" rather than something direct and easy to understand like "they feel bad that they don't fit, so they end up buying other things like makeup/jewelry to feel better about their appearance".
Simpler language is an accessibility issue
The average person has a hard time reading and fully understanding a newspaper article or cooking instructions on a pre-prepared meal.