I think it just adds +55% armour absorption (so only versus projectiles). It doesn't boost anything that already exists.

Adding to this subject somewhat I've been wondering what is the difference between various hull size in your sips like like small, medium or large? I get the some use more command points but when hovering over an hull type symbol it also talks about hull armor bonuses or something like that...

Edit: Found the detail information toggle on the ship information screen so I've figured it out.