You may notice your hull build exceed its maximum weight when using a single armor special, like in the screenshot below:
It can be confusing why a single Armor special, like Reactive Armor III would add so much weight to the hull.
All specials will increase the weight of a hull. In the case of Weapon or Engine specials, this is usually a percentage of the weapons equipped or the total weight of the hull.
Armor specials are unique and will increase the weight of a hull by a percentage of the total armor points a hull has. The more armor a hull has, the more weight will be added by an armor special.
Looking at Reactive Armor III, we can see that the percentage by which the hull's weight will increase is 15% of the total armor:
Since the Ifrit has 2,327,500 Amor Points, the weight added to the hull will far exceed the 20,000 tons it is capable of using. There are some additional calculations that are involved in weight formulas, but generally a large armor pool will not allow for these specials to be used.
In most cases, hulls that have large armor pools are not designed to need or use an Armor Special like this. They will usually have abilities to augment their defenses without needing the extra special's weight increase.