Perhaps chubby girls see long hair as a means to compensate for a body shape that they see as a disadvantage when trying to attract men, so simply more of them (more than skinnier girls) rely on the effect that long hair has on average has on men, and it may just so happen that more chubbier girls choose to grow their hair out.

But the correlation of chubbiness with more frequent long hair is an observation I have not made myself. I´m just trying to find reasons here for the observation you presented in your post.

If a chubby girl manages to lose a lot of weight, I cannot see how that should possibly have the slightest effect on her hair growth and quality.