Compress a DMG on macOS

A .dmg is a disk image — a file containing a whole filesystem. They’re how Mac apps get distributed outside the App Store, and they have a habit of being larger than the thing inside them.

What you do about that depends on what you’re actually trying to accomplish.

“I just want the files out of it”

You don’t need to compress anything. Drag the .dmg onto Purrchiver and it lists the contents like any archive — no mounting required. Extract everything, pull out a single file, or drag entries straight to Finder.

This is faster than the usual dance of double-click, wait for it to mount, copy, then remember to eject. Encrypted DMGs prompt for the password first.

Free, no unlock.

“I need to send it to someone and it’s too big”

Wrap it. A DMG is just a file, so any compressor works on it:

ZIP — maximum compatibility. The recipient double-clicks it. Free in Purrchiver.
7z or Zstandard — noticeably smaller, at the cost of the recipient needing something that reads it. Part of the unlock.
age — if it needs to be encrypted in transit, see age encryption. A .dmg.age is a thing Purrchiver makes and most Mac tools can’t.

How much you gain depends entirely on the DMG’s contents. An image that’s already compressed internally (most app DMGs are) will barely shrink. One full of documents or raw assets can shrink a lot.

“I’m making a DMG and want it small to begin with”

Pick DMG in Purrchiver’s Create Archive sheet to bundle a selection into a disk image. Purrchiver builds it through Apple’s own hdiutil, so what you get is a standard UDIF image — nothing proprietary, nothing that behaves oddly on someone else’s Mac.

Plain DMG creation is free. An encrypted (AES-256) DMG is part of the one-time $9.99 unlock.

If you’re distributing an app and size is the priority, the honest answer is that a compressed DMG and a ZIP of the app bundle are close enough that the choice should come down to presentation. DMGs let you set a background image and an Applications symlink; ZIPs don’t. Pick on that, not on bytes.

“The DMG is huge and mostly empty”

This is the case people usually mean by “compress a DMG,” and it’s worth naming: a disk image made at a fixed size stays that size even when the filesystem inside it is mostly free space.

Converting it to a compressed format reclaims that space for transport, because the empty regions compress to almost nothing. Wrapping the image in ZIP or Zstandard is the simplest route, and it’s non-destructive — you keep the original.

When to just use Finder

If you want to look inside a DMG and you’re not in a hurry, double-clicking to mount it is fine and always has been. Purrchiver is the better tool when you want files out without mounting, when the DMG is encrypted, or when it’s one of a pile of mixed archives you’re working through.

Bootable DMGs are a genuine exception — Purrchiver reads data images, not bootable ones. Mount those in Finder.

Get Purrchiver on the Mac App Store →