A drink served over one large cube and the same drink served over crushed ice behave completely differently, even with identical ice by weight. The variable is surface area, not quantity.
Melting happens at the surface
Heat enters ice through the boundary between ice and liquid, so the rate of melting depends on how much boundary exists.
Dividing a block into smaller pieces multiplies that boundary while keeping the mass unchanged.
Crushed ice therefore dilutes a drink several times faster than a single cube of the same weight, which is a geometric consequence rather than a matter of ice quality.
More ice can mean slower dilution
Filling a glass to the top with ice sounds like a route to a watery drink and usually produces the opposite.
A full glass keeps the drink colder, and colder liquid melts the remaining ice more slowly than a half-filled glass of warmer liquid.
A drink with three cubes floating in a warm glass will end up more diluted than the same drink packed with ice.
Clear ice is denser than cloudy ice
Cloudiness comes from trapped air and dissolved gases that were pushed ahead of the freezing front and never escaped.
Those inclusions create internal weak points and increase the effective surface once cracking begins, so cloudy ice breaks up and melts faster.
Directional freezing, where water freezes from one side and impurities are driven into a discarded portion, is the standard way to produce clear ice.
Crushed ice is used deliberately
Fast dilution is exactly what some drinks want. A strong, sweet, spirit-heavy build served over crushed ice opens up as it melts.
Those drinks are usually built with extra spirit to account for the dilution they will receive over the time they are drunk.
Pebble ice, which is softer and holds liquid within its structure, dilutes faster still and is chosen where a drink is meant to change noticeably between the first sip and the last.
The trade is a short window at its best, which is why such drinks arrive with a straw and are expected to be finished rather than nursed.
What temperature the ice arrives at
Ice held at a low freezer temperature is colder than freezing point and cools a drink for a period before it begins melting meaningfully.
The same ice left in a warm bar is already melting and delivers dilution from the first second of contact.
Bars that care about consistency treat ice temperature and dryness as a controlled input, in the same way a kitchen controls oven temperature.