Understanding Zip File Size Differences

What will you learn?

In this detailed guide, you will uncover the reasons behind discrepancies in zip file sizes compared to their original sources. You’ll gain insights into how compression methods, file content, and metadata influence the final size of a zipped file. By understanding these factors, you will be able to manage your expectations regarding compressed files’ sizes more effectively.

Introduction to the Problem and Solution

Encountering a situation where the size of your newly created zip file differs from an existing one can be perplexing. This discrepancy is often attributed to various factors such as the compression algorithms used, the nature of the data being compressed, and additional metadata within the archive. Compression methods play a crucial role in determining the ultimate size of a zipped file, with different algorithms exhibiting varying levels of efficiency based on the data being compressed.

To address this issue, we delve into how compression functions and what factors impact its effectiveness. By exploring these aspects, we aim to shed light on why zip file sizes differ and how different types of data and compression settings contribute to these variations. This knowledge equips you with a better understanding of how to manage expectations when working with compressed files.


import os
import zipfile

def create_zip_file(source_path, destination_zip):
    Creates a ZIP archive from a specified directory.

        source_path (str): The path to the directory to be zipped.
        destination_zip (str): The path where the output ZIP should be stored.
    with zipfile.ZipFile(destination_zip, 'w', zipfile.ZIP_DEFLATED) as zipf:
        for root, dirs, files in os.walk(source_path):
            for file in files:
                zipf.write(os.path.join(root, file), 

# Copyright PHD


The provided Python code illustrates how to create a zip archive using Python’s zipfile module. The create_zip_file function accepts two parameters: source_path, which points to the directory you want to compress, and destination_zip, specifying where you want to save your new ZIP file.

Key Points:ZIP_DEFLATED: Specifies deflation as the compression method due to its balance between speed and efficiency. – os.walk: Used for traversing directories and including all files in the ZIP archive. – Relative Paths: Ensures that only necessary hierarchical information is included by making paths relative when adding files into an archive.

While copying or re-zipping existing archives may not always result in identical sizes due to potential variations in metadata or differing compression levels applied each time.

  1. How does different content affect zip filesize?

  2. Different content types impact compression differently; text documents compress well while formats like JPEG images or MP4 videos are already compressed.

  3. Can changing compression level affect filesize?

  4. Adjusting compression levels can alter filesize � higher levels usually yield smaller files at the cost of increased processing time if supported by your tool.

  5. What role does encryption play?

  6. Encrypting contents within a ZIP may slightly increase overall filesize due to added security features.

  7. Does empty space or formatting inside files matter?

  8. Files with whitespace or redundant data see significant reductions after being compressed.

  9. Will every tool produce exactly same sized zips?

  10. Various tools may yield different outcomes even with similar settings due to distinct approaches towards archiving/compression.


Understanding why there are differences in size among zipped versions of similar datasets primarily revolves around intricacies associated with data compression and archiving processes. Factors such as choice of algorithm and specific content formatting significantly influence these variations.

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