Traveling light is always a good idea. You are going on a vacation for enjoying, not for looking after those heavy bags.
Possessing an extremely large and bulky backpack on public transportation may seem fine, however, whenever the bus is crowded you cannot take it with you.
You'll have to walk with your bag, sometimes right throughout the whole city to search for a hotel and the weather can be very hot or very cold depending on what country you're visiting. You can read more about how to travel light by browsing the web.
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If you do take transportation, you can swing a tiny bag over the front and jump in a cab or rickshaw with no difficulty. Similarly, leaving your heavy bags in lockers may be a problem if you aren't packing light.
When utilizing mini-buses, they're often crowded and don't have any baggage holds, which means that your bags go forward with you. If crowded you'll hit everybody with your luggage when locating a chair and may want to have an extra seat for your big luggage.
You'll be uneasy moving from village to village and also being unable to just jump on and off the small buses. Packing light will make your life simpler while traveling from city to town.