This exploit allows your companion (except dogmeat) to hold as many items as you want them to carry there is no limit! If you want them to carry every item in the game they can do it!
First off drop whatever items you want for them to carry for you. Then simply ask them to pick up the items for them to hold for you. Even if they cant carry anymore items they will gladly pick them up and put it in their storage.
This exploit is perfect for hoarders that don't want to store anything anywhere else or if you are carrying to much weight on you and don't want to discard anything.