I took the liberty of changing the title. The original was the slightly clickbaity "Are Linux Programmers Getting Too Fat?".
Bad correlation. L/XL(XXL also in many cases) doesn't necessarily mean the person is obese. XL is a common size for people with a moderate to tall height and good shape.
The shift is still there, though. The data missing, though, is the sizes in cm/in. I wouldn't be surprised if M/L/XL changed meaning between those dates.
It is possible that the meaning of the sizes changed, but that would probably go in the direction of making the shift larger. [1]