Does Gzip Compression Increase AVIF File Size?

Applying Gzip compression to an already compressed AVIF file will almost always increase the total transmitted file size rather than reduce it. Because AVIF files are pre-compressed using sophisticated, high-efficiency codecs, their data contains virtually no compressible redundancy. When a web server forces Gzip on an AVIF image, it adds compression headers, block markers, and checksums to high-entropy data, resulting in a larger payload and wasted computing resources.

Why Double Compression Fails

Compression algorithms like Gzip rely on the DEFLATE algorithm, which combines LZ77 (finding duplicate strings of bytes) and Huffman coding (assigning shorter codes to frequently occurring symbols). For DEFLATE to shrink a file, the target data must contain repeating patterns or an uneven distribution of bytes.

AVIF (AV1 Image File Format) relies on the AV1 video codec's intra-frame compression. It already uses complex discrete cosine transforms (DCT), directional prediction, quantization, and asymmetrical numeral systems (ANS) entropy coding to strip away all structural and statistical redundancy.

By the time an AVIF file is generated:

• The byte distribution is nearly uniform.
• The data exhibits maximum information entropy, closely resembling random noise.
• There are virtually no repeating byte sequences for LZ77 to reference.

The Source of the Size Increase

When Gzip attempts to compress high-entropy data like an AVIF image, it cannot find matches to build its sliding dictionary. Instead of reducing data, Gzip must emit literal bytes wrapped in framing overhead.

Every Gzip stream must include:

• A 10-byte header (magic numbers, compression method, flags, timestamp).
• Optional metadata (original filenames, extra headers).
• Deflate block framing (dynamic Huffman tree structures or non-compressed block headers).
• An 8-byte trailer containing a CRC-32 checksum and the uncompressed input size modulo \(2^{32}\).

Because the underlying image data cannot shrink, these mandatory headers and framing bits are added directly to the original file size. As a result, the transmitted file size grows by anywhere from a dozen to several hundred bytes.

Negative Impacts on Web Performance

Serving Gzipped AVIF files harms performance in three distinct ways:

1. Wasted Network Bandwidth: Every request delivers a slightly heavier payload over the network, negating the efficiency gains of using modern image formats.
2. Server CPU Overhead: The server wastes processing power attempting to compress data that cannot be compressed, reducing server throughput under heavy traffic.
3. Client Latency: The client's browser must allocate memory and CPU cycles to decompress the Gzip layer before passing the underlying data to the image decoder, delaying the Largest Contentful Paint (LCP).

Best Practices for Server Configuration

Web servers should only compress text-based or uncompressed assets, such as HTML, CSS, JavaScript, JSON, XML, and SVG. Binary media formats—including AVIF, WebP, JPEG, and PNG—should always be excluded from HTTP compression modules like Gzip and Brotli.

To prevent this issue, ensure your server configuration restricts compression strictly to compressible MIME types:

• Nginx: Ensure image/avif is omitted from the gzip_types and brotli_types directives.
• Apache: Use mod_deflate with filters that explicitly exclude image formats, or use SetEnvIfNoCase Request_URI \.(?:avif|webp|jpe?g|png|gif)$ no-gzip dont-vary.
• Content Delivery Networks (CDNs): Most modern CDNs bypass Gzip and Brotli compression for image MIME types by default, but custom compression rules should be audited to verify that binary image extensions remain uncompressed.