The people who never check a bag aren't packing less than you — they're packing differently. The system matters more than the discipline. Most overpacking isn't really about bringing too many things; it's about bringing things in a shape that wastes space.
Rolling clothes instead of folding them is the well-known trick, and it does help, but it's not the biggest space-saver. The bigger shift is separating your bag into zones with packing cubes — one for tops, one for bottoms, one for everything else — rather than one open compartment where everything shifts and resettles every time you dig for something at the bottom.
The actual system
Lay out everything you think you need, then remove a third of it before you even start packing. Almost everyone overestimates outfit changes and underestimates how much they'll just re-wear the same two or three combinations. Pack by outfit, not by category — it's much easier to spot "I have four things that only work with one other thing" when you lay out full outfits instead of a pile of tops and a separate pile of bottoms.
What actually goes in the bag
Heavy, rigid items (shoes, toiletry kit, electronics) go at the bottom near the wheels, where the bag's structure can support the weight. Soft, compressible items (that packing cube of shirts) go on top, where they can flex into whatever space is left. This single ordering habit is why two people can pack an identical pile of items and one ends up with a bag that closes easily while the other is sitting on it.