I think it has to do with the time you leave it on there...

I once gave it to my Rayquaza and left it there for a few days in my party (It became my designated FLYer) using it in battle here and there...when it finally reached level 71, its stats exploded (7 Special Attack points in one level?... ) I think it slowly but surely increases the amount of stat points you'll gain when a pokemon levels up. Putting the Macho Brace on right before it levels up does absolutly nothing. It takes about a day for a stat to go up by one point due to the Macho Brace. If it allowed double points when it leveled up at the cost of only 3/4ths of its speed(At ;east that's what I think, considering Rayquaza was unable to outrun a Walrein and Registeel at level 51 couldn't even escape from a Magikarp...), it would be a little too cheap...