How a few kilobytes turn into a huge workload
A ZIP file is small because compression stores repeated patterns efficiently. If a file contains the same simple pattern over and over, the archive can describe that pattern compactly rather than writing every copy out. When an unzip program reverses the process, it must recreate all the data the compact description represents.
Some zip bombs amplify the effect with layers of archives: unpack one file and it reveals another archive, which expands again. The original download can look modest even though processing it demands far more storage, memory, and time.
Where the crash happens
The danger is not that the ZIP file magically damages a computer. It is that an application may start extracting or scanning the contents before it has a reliable picture of how large the expanded data will be. As files are written or examined, available disk space or memory can run out; the application may freeze, fail, or crash.
The same pressure can affect security software that inspects attachments, since it may need to unpack files to check what they contain. A protected system can limit the total amount extracted, stop processing after too many nested layers, or reject archives whose expansion exceeds a safe threshold. Those checks matter because the risky part is often invisible until the archive is opened.
