Why should Big Men tuck in their shirts?
Fashion is often dictated by circumstances.
In the USA, the habit of not tucking your shirt into your trousers came about due to the number of obese men in that country, wanting to hide the situation.
The fashion amongst young men in the USA of wearing "low crutch jeans" came about as they admired the look of contemporaries in jail who were not permitted belts, so their prison trousers always hung off their hips.
There was a common rule amongst many golf clubs that shirts worn on the course must have a collar and be tucked into trousers. But that seems to have gone out of the window, since Tiger Woods wore a collarless shirt in a competition.
Jeans were not permitted to be worn in my clubhouse until about ten years ago.
The women of course were allowed "to wear whatever they choose."
This always amuses me. Ian Botham who is clinically obese, "more worried about his legs than his huge girth," the shirt tails outside his trousers hides nothing!