There is no rule against it. It means that the weapon itself will be just as powerfull as the real deal, if the robot frame or armour can take the load that is.

However, I would advise on making up your own specifications, and base them on existing robot weapons. For instance, Gravity 3 (the one in series 7) has a 100mm x 180mm ram. you could make yours 120mm x 180mm, that makes it more powerfull (5.5 tonnes as opposed to 4 tonnes) without too much of the downside (weight).

You could do the same on all kinds of vapourbots. Since vapours do not keep account of things real robots must take into account (metal fatigue, reliability, survivability, ease of maintenance) vapoursbots can get away with a lot more than real robots can. So real robots tend to keep the powers more in check a bit.