That's an excellent and very observant question! Yes, you are absolutely right. Many cities worldwide are indeed implementing public trash cans with specifically designed small openings. The primary goal is exactly as you guessed: to discourage people from disposing of oversized bags of household or commercial waste in public bins, which is a common form of illegal dumping.
These bins typically have an opening just large enough for standard drink bottles, food wrappers, and small debris—the typical litter from pedestrians. This design effectively pushes the responsibility for larger waste back to proper residential or commercial collection services. It helps keep public bins from overflowing prematurely, reduces maintenance costs, and improves overall street aesthetics.
Beyond size restriction, this design often works in tandem with waste sorting initiatives. You might see separate slots for "Recyclables" (like cans and bottles) and "General Waste," each with a limited aperture. This not only controls what goes in but also encourages people to think for a second about what they are discarding.
From a practical standpoint, it's a simple yet clever nudge in behavioral design. While it might be slightly less convenient if you have a large coffee cup, the broader benefit for community cleanliness is significant. Cities from London to Tokyo have adopted variations of this design, finding it a key tool in maintaining cleaner urban environments. So next time you see one, you'll know its specific purpose in the ongoing effort to manage urban waste smarter.