A bank of storage lockers at a station or airport is a rental of fixed volume for a fixed period. The pricing structure follows directly from that, and it varies in ways that tell you what the location is built for.
The unit being sold is cubic space
A locker bank is a fixed wall of compartments, and the operator cannot add capacity in response to demand the way a staffed room can add shelving.
Size tiers exist because a large compartment consumes the space of several small ones, so the price steps up faster than the volume does.
This is why the largest sizes look disproportionately expensive. They are, and the pricing is discouraging their use for small bags.
Time blocks encourage turnover
Most banks charge by a block of hours or by a calendar day, with the meter continuing until the door is opened.
Rates commonly step up after the first day, since a locker holding a bag for a week is producing far less revenue than one turning over daily.
Maximum durations exist for the same reason, and abandoned items are typically removed to a holding area after that limit.
Staffed counters price on a different axis
A person behind a counter can stack irregular items, accept skis and boxes, and use the full height of a room, so the constraint is floor area rather than compartment count.
Counters therefore price per item per day, which favors a traveler with one very large bag and penalizes a group with many small ones.
The choice between the two usually comes down to shape: lockers for regular suitcases, counters for anything that will not fit a rectangle.
Location determines the tier mix
A bank at a rail station serves connecting passengers with a few hours, so it holds mostly medium compartments and prices in short blocks.
A bank near a cruise terminal or a major attraction expects full-day storage of large suitcases and carries more oversized units.
Reading the mix before arriving saves the common failure, which is finding only small compartments free when carrying a checked-size bag.
The alternatives have different failure modes
Hotels will usually hold bags for their own guests on arrival and departure days, which covers the most common need at no cost.
Shop-based storage networks, where a café or a store holds bags for a booked slot, depend on that business being open and staffed at the hour you return.
The reliability question in every case is the closing time, since a locker bank inside a station closes with the station and a shop closes with the shop.