I believe the intention is to apply some form of common sense to this. Such as expecting a rocket launcher that fires an anti-tank missile to kill a tank. The limitation however would be that the missile and launcher would probably be heavy and cumbersome so while a soldier might be able to carry something that can hit well above their class, it has limits.
For example, say a soldier out of armour wants to shoot down a shuttle. Their best bet is going to be a missile system, however since they don't have the benefits of a PA, the best they're probably going to get is a single shot launcher. So they have to carry the weapon out to a vantage point, aim at the shuttle they intend to down, and fire. This is where the GM's discretion would come in, the GM has to decide whether the shuttle is capable of defending against a missile that could theoretically down it, does it have good shields? Perhaps a point defence system exists on the shuttle, or maybe the pilot is good enough to out manoeuvre the missile. Either way it comes down to the problem that while the lone soldier might possibly shoot down the shuttle with his one missile, he only has the one chance. Given that a shuttle probably carries a PA grade weapon fed by the vessel's power plant, chances are that the lone soldier won't last long.
More or less what I've said in a long winded way is that weapons that can hurt larger targets will probably have drawbacks that prevent them from breaking balance. Additionally I believe that what makes a larger platform more dangerous isn't necessarily bigger guns, but rather more diverse and more effective options like sensors, speed, and other tertiary systems. We should take into consideration how the weapon will be employed as much, if not more than what it will kill if you line up a shot.