It's just my way to do or what i found on forums

If you have a better or easier way to do it, i will very enjoy to know it ;)

1 - Select your button, in the Inspector > Button > Transition, select Animation

2 - Click on Auto Generate Animation

3 - Save your animation (in your folder animations)

4 - Open the window Animation (if the windows is not open) and click on the record button (make sure your'e on Normal)

5 - Change the Z axis to 1 > 0 (Inspector Button) and rechange 0 > 1 (just for the animation begin at normal button size) and change Normal > Highlighted

6 - Click on the record button (Animation) and change the size of your button

- (Optional) Open the Animator window, click on the arrow Normal and change le value of Transistion Duration (Settings)

- Do the same for the arrow Highlighted

7 - Click on Play and pass your cursor on the button

(out)

(in)

> Short video :

8 - Don't forget to change the value (Inspector Button) of what you want (pressed, selected, ...). In my case, put Highlighted for Highlighted Trigger & Selected Trigger)

9 - ADD SELECTED FIRST (Navigation UI)

- You can add a "selected first" option, in the Event System, drag & drop the button you want :

- Set the Navigation for all buttons

- Click on Visualize for a better view

10 - PAUSED 

- If you want to make pause menu with animation, don't forget in the Animator (Inspector) to change Update Mode : Unscaled Time (for all buttons)

- Thanks to the forum again