I've built a few custom combat vehicles going all the way to the maximum once I became aware of it (and they're appropriately tough for their respective weights), but it makes sense that the limits would have been consciously set so that no extant canon designs goes over. That also means that I don't normally worry about exceeding them by accident; between tonnage limits and the way my ideas of what constitutes a "proper" balance between armor, mobility and firepower tend to work out, I just never end up getting there unless it's on purpose.