Interesting idea, but if I play it out:
The average may be the same but the range seems to break it.
1.5 times 1 is 1, times 2 is 3, times 4 is 6, times 5 is 7.
And, in the rare event of negative Strength modifiers, x1.5 takes away damage.
I think sticking with the already in use Enlarge spell...