Mac Dock resizes when you connect a monitor

Why it happens

The Dock has one size setting for the whole Mac, not one per display. tilesize is a number of points, and a point is not a fixed number of pixels. On a Retina laptop screen a 48 point icon is drawn across roughly twice as many pixels as on a 1080p monitor, so the Dock looks correct on one and wrong on the other, without the stored value ever changing.

That is why the Dock appears to resize itself the moment you plug in or unplug a monitor: the number stayed put, the canvas changed underneath it.

The part that really does change

There is a second, separate problem, and this is the one worth locking. When the Dock lands on a screen of a different size, its maximum width changes with it. A Dock that fits comfortably across a 27 inch monitor does not fit across a laptop screen, so macOS shrinks the icons to make them fit. Come back to the big monitor and they do not always return to where they were.

Every accidental brush of the divider while it is on the wrong screen writes a new tilesize, and that value is the one that follows you back.

What to do about it

1. Set the size you want, once

Pick the size on the display you use most, then read the value so you can put it back later:

defaults read com.apple.dock tilesize

Write that number down. It is an integer, normally between 16 and 128.

2. Lock it so nothing can write a new one

defaults write com.apple.dock size-immutable -bool true
killall Dock

The size slider in System Settings stops responding and the divider stops resizing. The Dock still scales visually between displays of different densities, because that is macOS drawing it, but the stored value can no longer drift. To release it, swap true for false.

3. Decide which screen it lives on

If the Dock jumping between monitors is the real annoyance, size is only half of it. See how to keep the Dock on one screen, which covers what macOS does and does not let you pin.

What does not work

• A per display Dock size. macOS has no such setting. Anything promising one is either resizing the shared value as you switch, or replacing the Dock entirely.
• Setting tilesize above 128. The Dock accepts it from the command line and then clamps it on the next launch. It is not a hidden larger size.
• Changing display resolution to compensate. It works, and it costs you screen space everywhere else. Locking the size is the smaller change.

Putting it back

defaults write com.apple.dock size-immutable -bool false
defaults write com.apple.dock tilesize -int 48
killall Dock

Use whichever number you noted in step one. If you never noted one, defaults delete com.apple.dock tilesize returns the Dock to the macOS default rather than a guess.

Does this affect magnification?

No. size-immutable pins tilesize only. Magnification and largesize stay adjustable unless you lock them separately, and locking the size while magnification is on is a common way to get a Dock that still looks like it is moving.

Will locking the size break screen sharing or a projector?

No. The lock stops the value being written; it does not stop macOS drawing the Dock at whatever scale a display needs. Presenting works exactly as before.

Which macOS versions does this apply to?

All of them that matter. size-immutable has worked for over fifteen years and still works on macOS 26. The settings pane moved to System Settings › Desktop & Dock, which is where the grayed out slider will be.

If you would rather not keep a list of commands, Still Dock is the same four settings as switches, and it remembers the Dock you had before you locked it. The size lock is free.