"Once you find your seat on board, place your larger carry-on baggage in the overhead compartment above your row and your smaller personal item under the seat in front of you, which helps everybody have space to store their bags."
I suppose you could make an argument that 'smaller' and 'larger' are making a comparison to the average size of all bags, but that seems like a stretch. Your largest item can go in the overhead, even if it is smaller than the maximum allowed carry-on size. The phrasing really doesn't imply that the under-seat storage should be your primary storage space. It's more of an appeal to only stow one item in the overheads.