The cost of a heavy bag is not felt at the airport. It appears repeatedly across a long trip, in decisions that get made differently because carrying the bag is unpleasant.
Weight decides which options stay open
A traveller with a light bag can take a local bus, walk from the station or accept a room up three flights of stairs without much thought.
The same choices become genuinely unattractive with a heavy load, so the traveller takes taxis, books closer to the centre and pays more for convenience.
Those decisions repeat several times a day for the duration of the trip, which is how a few extra kilograms turns into a meaningful expense.
Bag size sets its own contents
People generally fill whatever container they bring, so the initial choice of bag determines the final weight more reliably than any packing list.
A smaller bag forces the decisions that a larger one defers, and it does so before departure when there is still time to reconsider.
This is why experienced travellers select the bag last, after deciding what is genuinely needed, rather than packing into whatever is already owned.
Duplication is the usual source of excess
Long trips tempt travellers to pack for every scenario, which produces multiple items serving overlapping purposes with different specialisations.
The practical alternative is one item that works adequately across conditions, accepting slightly worse performance in each for a substantial reduction in total volume.
Consumables are the exception worth carrying, since specific medications, contact lenses or documents may be genuinely hard to obtain along the route.
Almost everything can be bought en route
Most destinations sell clothing, toiletries and equipment, frequently at lower prices and better suited to local conditions than anything packed at home.
Buying on arrival also defers the decision until the actual conditions are known, rather than guessing months earlier from a forecast.
The mental barrier is treating a purchase abroad as an admission of poor planning, when it is often the cheaper and lighter approach.
Laundry is the variable that resolves it
Packing volume is largely a function of how long the traveller intends to go between washes, and shortening that interval collapses the requirement.
Laundry services are inexpensive across much of the world and take a few hours, which removes the need to carry a fortnight of clothing.
Building one laundry stop into a longer itinerary is therefore a packing decision as much as a scheduling one, and it usually saves more weight than any other single change.