It is definitely possible. Gorillas are estimated to be 6 to 15 times as strong as humans. If gravity has a direct and 1:1 ratio on evolutionary trends towards strength (big assumption here) that means the 0.5g gorilla would have 3 - 7.5 times the strength of the 1.0g human. They would still be slightly bigger.
So definitely possible. However I have the feeling that strength would be less of a trending factor in evolution on a low G planet.
Alternatively, lower G might mean fewer calories are consumed per exertion so greater levels of strength can be more successful.
We need a larger sample size to say for sure. But it is definitely possible.
Well that is all a matter of theory/opinion at this point. But why is strength a positive adaptation? It lets you perform better at the tasks that Jason mentions. However with 2 creatures of equal mass, and all other conditions being equal, less strength is required in a 0.5 g planet to perform the same feats of strength (in most cases). To out perform a creature to the same degree as on a 1.0 g planet requires less muscle mass and strength when you are on the 0.5 g planet. This changes the effect of strength variation on overall survival.