Just like everything else when a rule is made someone will circumvent it to take an advantage for the wrong reason. It's a shame when a fighter makes weight at 158 pounds and then fights the next day at a weight of 175. This happens in every championship fight.
I'm sorry but if the weight class is 158 the that is the weight that must be met when the fight takes place. Unfortunately. boxing is driven by the dollar and if a fighter does not make weight before the fight then the bout is not official and all bets are off. Too bad!! If you want to be a true middleweight champion then fight as a middleweight and not a light heavy weight the night of the fight.
When a true middleweight comes in and his body is developed for that weight he will not gain much weight. Many fighter deplete their bodies to make the weight then pile on 10 - 15 pounds in one day to bloom up sometimes even much more. If the natural middleweight only gins 3-5 pounds then it becomes unfair and even very dangerous.
Here is a recent example from 10/17/15
When the fight took place each fighter weighed over 170 pounds!
More proof -
Updated 11/8/15 - Bradley vs Rios
Bradley set the pace for the fight right away, landing a series of head shots early in the first round. Rios, who ballooned up in weight after Friday's weigh-in, plodded forward trying to land a big punch but was never able to land more than one shot at a time.
Rios, who had to weigh in twice the day before the fight after coming in slightly over the 147-pound limit, weighed in unofficially at 170 pounds on the HBO scale in his dressing room. Bradley, who weighed 146 at the official weigh-in, was 155 in his dressing room