Size and heaviness are separate
Why Is a Big Thing Not Always Heavy?
Weight depends on how tightly packed a material is, not on how much room it takes up.
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Step 1 of 5
Size and weight answer different questions.
Size asks how much room something takes up. Weight asks how strongly gravity pulls it. Nothing forces those two to agree.
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Step 2 of 5
What matters is how tightly packed it is.
Metal packs a great deal of matter into a small space while foam is mostly trapped air, so equal-sized lumps weigh wildly different amounts.
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Step 3 of 5
The same size can differ enormously in weight.
A brick-sized block of foam weighs almost nothing while the brick itself is a struggle to lift, despite occupying identical space.
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Step 4 of 5
A trick question hides in the classic riddle.
A kilogram of feathers and a kilogram of lead weigh the same, because both were measured out to a kilogram. Only their size differs.
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Step 5 of 5
Weight is a pull, so it changes with location.
Weight is gravity's pull on your matter. Take the same object to the Moon and it pulls six times less, though nothing about it has changed.
A litre of water weighs one kilogram, a litre of air about 1.2 grams, and a litre of lead over 11 kilograms.
The short version
Big does not mean heavy, because weight depends on how tightly matter is packed into a space rather than how much space it fills.
Try it yourself
Hold a large bag of crisps in one hand and a small tin in the other. The far smaller object is obviously the heavier one.
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