In boxing and in wrestling, only the upper weight limit is given in Wikipedia and other sources that I have found. This would mean that a flyweight fighter can compete in the heavyweight division and not vice versa. Is it actually legal that a flyweight fighter be allowed to fight in heavyweight division in boxing or wrestling?
According to the Canadian Amateur Wrestling Association, which I think follows the same rules as the Olympics:
A wrestler can choose to compete in the next higher weight class, except for the heaviest weight class, where the wrestler must weigh the minimum weight listed in this rulebook. The decision to change a weight class cannot be made after the wrestler has already weighed in and the athlete has officially been entered on the weigh-in sheet for a particular weight class.
Not true. A fighter can legally only fight someone in the same weight class. So every division has a minimum even if it is not stated. A fly weight cannot fight a middle weight on normal circumstances. Middle weight fight middle weights, heavy weights fight only heavy weights. This also implies size matters in a sports fight setting. The stronger one will win of the skill sets are really close enough.