Was reading Which planet or moon would be most feasible to terraform? and specially https://space.stackexchange.com/a/206/12379 . I am not really good at science or maths so please excuse below and look at the figures and ideas shared as a simple person's analogy to what he thinks.
While I don't know if we have technology to add mass to a planet. Let's say we decide to go for it so to have somewhat like Earth's gravity (from 1/5th say it becomes 1/3rd or better due to doubling size of Mars as well as having more real-estate for Mars colonists) while also presuming or assuming that such technology is not expensive to sustain (which probably is/would be wrong) wouldn't it change the rotational planet of Mars itself vis-a-vis in relation to other planets due to its attraction and repulsion by it, thereby also changing rotations of other planets, which will have unknown ripple effects on the solar system as a whole (at least the nine planets that are there ?), maybe also changing/risking Earth's atmosphere in the process for the worse?
This is a what-if fantasy idea probably might have tried in some sci-fi B-Movie.