The aegis gains the powerful build trait while wearing his astral suit. Whenever the aegis is subject to a size modifier or special size modifier for a Combat Maneuver Bonus or Combat Maneuver Defense (such as during grapple checks, bull rush attempts, and trip attempts), the aegis is treated as one size larger if doing so is advantageous to him. The aegis is also considered to be one size larger when determining whether a creature’s special attacks based on size (such as grab or swallow whole) can affect him. The aegis can use weapons designed for a creature one size larger without penalty. However, his space and reach remain those of a creature of his actual size. The benefits of this trait stack with the effects of powers, abilities, and spells that change the subject’s size category.
Your brain armor basically gains the Titanic special quality. The armor's size being treated as larger doesn't provide any benefits to AC, and is exclusively related to calculations with size-based differences.
It is unrelated to Soulknife's weapon increase, as that ability affects things you do directly, while the Aegis' Powerful Build affects things that happen to you.