A single large suitcase looks like the efficient choice and usually is not. Splitting the same volume across two smaller bags changes what happens when a trip goes wrong.
Weight limits are per bag and per person
Airlines apply a weight ceiling to each checked item, and excess charges climb steeply once that ceiling is passed. A single case takes the whole allowance and hits the limit first.
Two bags of moderate weight often travel within the same total allowance for no extra charge, depending on the fare. The same possessions cost differently depending on how they are divided.
Ground transport applies its own limits. Trains, buses and small ferries frequently cap the size of a single item rather than the total a passenger carries.
Separation limits the damage of a loss
When a checked bag is delayed, everything inside it is unavailable. Splitting essentials into a carry-on means a missing case becomes an inconvenience rather than an emergency.
The items that matter are predictable. Medication, documents, a change of clothes, chargers and anything difficult to replace at the destination belong in the bag that never leaves you.
Insurance follows the same logic. Policies typically exclude valuables from checked baggage entirely, which means an expensive item in the hold is uncovered by design.
Two smaller bags are easier to move
Carrying weight in two balanced loads is less strenuous than dragging one heavy case, particularly on stairs. Older stations, apartment buildings and old town streets rarely have lifts or smooth surfaces.
A single overloaded case also fails mechanically. Wheels and handles are rated for a load, and the failure usually happens on a cobbled street at the worst moment.
Two bags can be lifted separately into overhead racks and car boots. One heavy case has to be lifted whole, which is where most travel back injuries come from.
It changes how you pack a trip with legs
Multi-stop trips rarely need the full contents at every stop. A smaller bag can be taken on a short side trip while the larger one stays in storage.
Left luggage services charge per item, so a small bag stored is cheaper than a large one. Some accommodation will hold a bag between stays at no cost.
The division also enforces a decision about what is genuinely needed. Packing into a defined smaller space is a more effective constraint than a resolution to travel light.
Where a single case still wins
For a single-destination trip with a direct flight and a car at the far end, one case is simpler. There is no handling to speak of and the risks the split protects against barely apply.
Very long stays are also better served by one large case. The overhead of a second bag stops paying for itself when it will be unpacked once and left in a wardrobe.
The dividing line is how many times the luggage will be moved. Count the transfers, and the answer usually decides itself.