(For a practical example of this article's philosophy, click here)
I like to use as many metaphors as possible in my writing. It gives my writing vividity, an aura of meaningfulness, and a sense of adventure towards an unknown destination, which may or may not exist. For me, the term "soul" is a metaphor, since I do not actually believe in the existence of these eternal entities. There is no physical place in the body that could house such a thing. Feel free, of course, to try proving me wrong.
For me, the term "soul" can be equivalent to "life force," "motivation," or "essence of being." It is not something that stands on its own, without a parallel term.
Long ago, I was interested in the relationship between "soul" and "mental age." I believe that there may not be a correlation between physical age (the length of time one has lived) and mental age (one's childish or mature personality). There may be people who are very old but have a childlike attitude, and young people with a very strict attitude, with wisdom that exists beyond their physical age.
I would consider a "light soul" to be the personality of a child, not necessarily as an insult. People with such a life force are very adventurous, seductive, attractive, excited, carefree, and pretty much everything that may be considered social, innocent, and extroverted. A "light soul" is a child at heart.
On the contrary, a "heavy soul" is not necessarily a miserable or stressed entity. As I said before, they may be wise beyond their actual age. They may prefer intellectual matters over materialistic ones, are very self-disciplined, introverted, very mature, and may or may not lead an ascetic lifestyle, devoid of obsessions after pleasure, adventures, and vanity. They may also prefer solitary pursuits, and are independent by nature, even if melancholic at times.
A "heavy soul" can also be regarded as an "old soul". I also do not take that term literally.
Personally, I view myself as a hybrid between the two. I do lead a somewhat ascetic and solitary lifestyle, and I am wise beyond my physical age. However, I am also quite innocent, as I fail to understand the cause or functionality of impurity, crime, vengeance, and evil in our world. I prefer intellectual activities over social events, but I am also an admirer of physical fitness.
Part of being a philosopher is to be less innocent, for the philosopher seeks to understand, above all.
I believe that a composition of "light" and "heavy" virtues can lead to a healthy and serene life. It is good to be kind-hearted, but it is also important to care for intellectuality and avoid the circle of materialistic pursuits. There are many more combinations that are possible for a healthy physical and mental condition.
Regardless of its metaphorical aspect, I tend to believe that the soul was invented as a concept for people to explain to themselves what happens to a person after they die. It is a result of distinguishing between the person and their body, as if they were different objects. The truth might be that when we die, we simply shut down, like a phone that has run out of batteries. However, it is important to remain open-minded about the concept of souls, as it is a widely-accepted theory, along with other theories like the afterlife or resurrection.