No reason why a heavy worlder or transhuman might not be able to do it. . .or do it with some form of damping mount, like Vasquez and Drake use for their squad support guns in Aliens. . .But as the GM I get to have people on the streets scream in fear, run, and call the cops if a crew of disreputable types walk down the boulevard with a squad support scale weapon.
Shrug, in SM I've had a few incidents of people using ship mounted weapons on planetary targets. . .more common with smaller ships that can enter atmosphere and land, but I've had a few orbital bombardment moments arise. . .once you put tactical nukes or worse into play as weapons the PCs have control over "How big is too big" starts to cease to be a "can I carry it" issue, and more a "What are the legal and practical limits and restrictions on this technology" kind of issue.