A baseball is supposed to weigh 5.25 ounces (= 148.835 grams).
So, the new baseballs are 100*(.5/148.835) = 0.33% lighter.
Since the number of homers increased by more than 0.33%, I doubt that weight is the explanation.
(Maybe the elasticity of the baseball changed, which might explain the rise.)
Its weight and weight redistribution.