Wherever in the world I go, one thing is guaranteed:
People will keep asking how I manage to travel for months with a backpack that's smaller than their carry-on.
The answer is always the same:
It's not about packing less, it's about planning, and then purposefully packing what you actually need.
Me wearing the 7 kg backpack that I'm living out of for 3 months.
Over the past 11+ years I've narrowed my process down to 3 primary steps:
A quick research and prepping session to figure out what I actually need: Checking temperature, planning for activities, and keeping an eye on weather predictions are the big 3 research points.
Create a layered capsule wardrobe based on the research I did before, so my wardrobe is tailored for my trip. No fluff, just useful gear, and clothes that go well together.
Space-saving packing methods to compress my luggage as much as humanly possible. Rolls, packing cubes, and restricting my available space.
Just doing one of these steps will free up significant space in your luggage.