Most attempts to travel light begin by deciding what to leave out and end with a full bag anyway. The reliable approach reverses the order and starts with the container.
Capacity is filled because it exists
Packing is a process of considering items in sequence, and each individual item looks reasonable against the space remaining.
With capacity available, the marginal item is always affordable, so the decision to include it is made under no constraint at all.
A smaller bag forces every item to compete for space against something else, which converts a vague intention into a series of real choices.
The bag itself carries weight
Larger bags use more material, heavier frames, more zips and additional compartments, so an empty large bag can weigh several times an empty small one.
That weight is carried on every journey regardless of what is inside, and it counts against airline allowances in exactly the same way as contents.
Wheeled cases add a further penalty, with the wheels, handle and rigid shell often accounting for a substantial share of the empty weight.
Small bags change how a trip runs
A bag that stays in the cabin removes checked baggage fees, waiting at reclaim and the possibility of separation from luggage during a connection.
It also makes ground transport viable. Buses, local trains and shared taxis all become straightforward when the bag can be kept on a lap or overhead.
The knock-on effect is that lighter travellers take fewer taxis and more public transport, because the alternative stops being unpleasant.
Laundry replaces volume
The largest single category in most bags is clothing, and quantity is driven by how long the traveller expects to go without washing.
Washing in a sink or using a laundry service every few days decouples clothing volume from trip length entirely, so a two-week trip packs like a four-day one.
Fabrics that dry overnight make this practical, which is the actual argument for technical materials rather than any claim about performance.
The constraint has to be physical
Weight targets are easy to exceed at the last minute, because the final items are always small. A bag that cannot close is a constraint that cannot be negotiated with.
Packing the smaller bag first and only reaching for a larger one if it genuinely fails is a better test than deciding capacity in advance.
Most travellers who try this discover the smaller bag worked, which is why frequent travellers converge on cabin-sized luggage without being told to.