Travelling with hand luggage alone changes far more than what a traveller carries. It removes several constraints simultaneously, and the effects compound over a trip.
Checked baggage adds fixed time at both ends
Bag drop at departure and the wait at the belt on arrival each consume time that does not vary with how far the flight travelled.
On short trips that overhead is a large proportion of the journey, and it falls at exactly the points where a traveller is least able to use the time.
Removing it also removes the possibility of delayed or misrouted baggage, which is the disruption most likely to affect the first days of a trip.
Ground transfers become simpler
A traveller with one bag can use local buses, metros and trains that are impractical with heavy luggage, particularly where stations have stairs rather than lifts.
That converts an expensive taxi into an inexpensive train on almost every arrival, and the saving repeats at each city on a multi-stop trip.
Walking a short distance to accommodation also becomes reasonable, which widens the range of places worth staying in a compact old town.
Itinerary flexibility improves
Storing a single small bag is easy, so a traveller can check out and still use a full day before an evening departure.
Changing plans mid-trip becomes practical rather than daunting, since moving between accommodation costs almost nothing in effort.
The cumulative effect is that a light traveller says yes to more of the trip, which is the part that is hard to see before trying it.
What actually drives volume
Shoes and outerwear dominate the space in most bags, so decisions about those two categories determine whether everything else fits.
Clothing that shares a colour range and can be layered reduces the number of items required to cover a range of conditions.
Laundry is the mechanism that makes it work, and washing a small amount every few days is what allows a wardrobe of a few days to cover a trip of three weeks.
Where the approach does not fit
Trips with specialist equipment, formal occasions or small children impose requirements that cannot be reduced by careful packing.
Cold-weather travel raises the volume of everything, and the outer layers that must be carried do not compress far.
The useful question is not how little can be carried but which constraints a traveller wants removed, since the answer differs by trip rather than by principle.