A Compression Weapon is a variant of the Ignition Weapon. Compression weapons often resemble normal weapons, yet actually contain far more mass than their volume would normally allow for. Apparently these weapons can be "decompressed" to their full size, although this has not yet happened in the series.
A famous Compression Weapon, Mago, is said to be so big that when decompressed it could pierce half of the whole Tower. The Green April is another well known example of a compression weapon.
Notes and Trivia
Leon 3 had a compression weapon of unknown name and rank.