ELI5: How can a file become much smaller when zipped, but return to its original size after unzipping?

A file gets smaller when zipped because the zipper finds repeated patterns and squishes them into a shorter way of writing, and it becomes original size again when unzipped because the computer carefully puts all the pieces back in the same order.

Think of it like packing clothes into a suitcase:

• Unzipped is like clothes spread all over the bed.
• Zipped is like folding them neatly and pushing out the extra air.
• Unzipped again is like opening the suitcase and taking the clothes back out.

• It looks for repeated bits or easy-to-shrink parts.
• It stores them in a compact code instead of writing the same thing again and again.
• When you unzip it, the computer uses that code to rebuild the exact same file.