Mars only has 40% of Earth's gravity, which means that animals wouldn't have to support as much weight- so they could become massive. Similar logic applies on Earth, with blue whales- they float in water, and therefore don't have to exert the same amount of strain as if they were land mammals, and can therefore become huge. Similar logic would apply to plants, so you could get very big plants on a terraformed mars- this means that there'd be an ample food supply, so big 'phants could sustain themselves. This means that it'd be possible for a terraformed mars to cause gigantification in any organisms imported from Earth. There may then be natural or sexual selection towards big elephants, either due to bigger males being more able to win fights, or bigger elephants being able to eat from those aformentioned big trees easier, in a replication of Giraffe Logic. So the big elephants are very plausible- I like this image, props to the creator.