You can change the size of the zoom step through View Preferences.
To change the size of the zoom step:
1. Choose Preference ® Options,
View.
2. Click Window.
3. In the Zoom Step Size text box, type the size value. The Zoom Step Size value equals the amount of the current view to magnify or reduce the image by. (For example, 2.25 times the current view, or 1/2.25ths of the current view.)
4. Click Save. Click OK.
The zoom step value is defined for future documents. To use the same value for the active document, repeat the procedure using View, Options instead, or close and reopen the active document.