A surfboard can weigh less than a packed suitcase and still cost considerably more to carry. Baggage pricing for sports equipment tracks volume and handling difficulty rather than mass.
The hold is sold by space, not by pounds
A narrow-body aircraft's cargo compartments are shallow and awkwardly shaped, and bags are loaded loose by hand into a limited cubic volume. Filling that volume, not reaching a weight limit, is usually the binding constraint.
An item that is long and rigid cannot be stacked around. It occupies a channel through the hold that would otherwise have taken several suitcases, so it consumes far more sellable space than its weight suggests.
Weight still matters for balance and for total payload on longer flights, but for most single-aisle operations the compartment fills before it grows heavy.
Length limits come from the door and the compartment
Every cargo door has a fixed opening, and an item longer than the diagonal of that door cannot be loaded at all. Compartment ceilings are low, so a board can only lie flat along the floor.
This is why acceptance for long items varies by aircraft type rather than by airline alone. The same board that travels easily on a widebody may not fit the regional jet operating the last leg.
Carriers publish their own dimension limits, and those limits change, so the size a given airline will accept is worth confirming directly for the specific routing.
Handling adds real cost
Oversize items usually bypass the standard baggage belt system and are carried separately by ground staff, sometimes through a dedicated oversize counter. That is manual labor at both ends.
Fragile long items also generate damage claims at a higher rate than ordinary suitcases, and the fee absorbs some of that expected cost.
Why board bags are built the way they are
Damage in transit typically comes from point loads at the nose and tail rather than from crushing across the middle. Padding concentrated at the ends addresses the actual failure mode.
Multiple boards in one bag is common because carriers frequently price the bag rather than its contents, though the weight allowance for a single piece then becomes the limit.
Hard cases weigh more but survive rough handling better, which is the tradeoff a traveler is really making.
The rental alternative changes the arithmetic
Established surf coasts generally have rental operations near the main breaks, and a week of rental often costs less than round-trip carriage of a board.
Travelers who bring their own do so for fit and familiarity rather than economy, which is a reasonable trade for a long trip and a poor one for a weekend.