The latest version of Safari for OS X has an iOS-like behavior if you’re using an Apple Trackpad, Magic Mouse, or Magic Trackpad to browse the web. If you double tap on the trackpad or mouse, the view within the Safari window will zoom in, just like a double tap on your iPhone or iPad version of Safari zooms in to help you read the web page on the smaller screen.
If this annoys you, here’s how to disable it.
First of all, launch System Preferences from the Applications folder, your Dock, or the Apple menu. Once launched, choose the preference pane that applies to you–click either the Mouse or Trackpad icon.
In the Mouse preference pane, uncheck the box next to the Smart Zoom feature. If you’re using a Trackpad, either one on your Macbook or connected via Bluetooth, tap on the Scroll & Zoom button at the top of the Trackpad preference pane, and uncheck the Smart zoom function there.
Whichever you choose, you’ll no longer have Safari zooming in at you when you only meant to set your finger down on the mouse or trackpad. If you want to enable this feature again in the future, simply re-check the Smart zoom function in the System Preferences again, and things will go back to zooming in within Safari when you double tap on the trackpad or Apple mouse.
Via: Macworld Hints