To get unlimited inventory space for your companion, instead of trading with your companion, simply command them to pick up the item for you. Companions will never be overburdened using this trick. If your own inventory is too full, just drop some heavy objects on the ground and have your companion pick them up. This is especially helpful when you find some good loot but cannot carry more weight, or if you do not want to burden yourself with heavy weapons. This trick allows you to always bring some mini-nuke launchers (Fat Mans), rocket launchers, and mini-guns to fights without having to worry about their weight.