How to Increase File Size on Mac Without Software

On macOS you can increase a file’s size with Preview (resize or re-export an image), with Finder’s Quick Actions, or from Terminal. Preview is the practical choice for images and PDFs, but like the Windows equivalents it cannot target an exact number of kilobytes.

Method 1: Preview (images)

Open the image in Preview and choose Tools → Adjust Size. Set the units to pixels and enter larger dimensions, leaving "Resample image" ticked. Preview shows the resulting uncompressed size at the bottom of the dialog, which is a useful, if optimistic, guide. Then File → Export, choose JPEG, and drag the Quality slider — Preview shows an estimated file size as you drag.

Exporting a JPG as PNG is the blunt instrument: it commonly multiplies the size by five or more, because PNG stores the JPG’s compression artefacts losslessly.

Method 2: Preview (PDFs)

Open the PDF and choose File → Export. In the Quartz Filter dropdown, avoid "Reduce File Size" — it is the one that shrinks. To go the other way, export the pages as images (File → Export As → PNG at a high DPI) and reassemble them into a PDF by dragging the images into Preview’s sidebar and printing to PDF. The result is substantially larger, but the text is no longer searchable.

Method 3: Terminal

macOS has mkfile, so mkfile 100k padding.dat creates a file of exactly 100KB, and dd if=/dev/zero of=padding.dat bs=1024 count=100 does the same. Both create new files rather than extending existing ones, and appending their output to a real image or PDF risks producing something a strict validator rejects.

Watch the units on macOS

Finder reports sizes in decimal — 1KB as 1,000 bytes — while most upload validators use binary, where 1KB is 1,024 bytes. A file Finder calls 100KB is about 97.6KB to the server. Press Command+I on the file and read the exact byte count in parentheses, then aim slightly above any minimum. The KB vs MB article covers this in more detail.

The shorter route

The tool on this site runs in Safari or Chrome on macOS, takes an exact figure in KB or MB, and writes the padding into metadata areas the format reserves, so the file stays valid and the image is untouched. Nothing is installed and nothing is uploaded.