This poster is part of a larger ad campaign by Tokyo Metro, targeting undesirable behaviors seen in and around trains in the city. In this case, the poster is aimed principally at the backpackers who flood into Japan from near and far to climb Mt. Fuji in the summer, forgetting that they are not the only ones in the train car.
Luckily, since you will have an opportunity to send any bulky baggage from Narita Airport to Ikata, you will most likely avoid this situation; however, keep in mind that a bag (or meal, car, room, etc.) that seems like a normal size to you, may be perceived by others as unnecessarily large. Try to be aware of space-- your own and other people's.