Animate a cube width

I have a gameobject > cube which represents the player. What I would like to do is when the player jumps the cube contracts on its x axis and then returns to full width. A bit like its breathing in and then exhales as it jumps. Exactly the same as in the game "thomas was alone".

I tried to do it with a series of flags and if statements but it seemed a little over engineered and didnt work very well. What's the best approach here? Should I be scripting an animation? Should I just stop using the cube and use a sprite animator?

I'm very new to games dev' and programming in a game loop. So, I'm asking for good practice/approach answers. Apologies if it's not a great question.

If it helps this is one of the original attempts I coded before I rage quit:

 void Update()
 {
     if (grounded && Input.GetButtonDown("Fire1"))
     {        
         rigidbody2D.AddForce(transform.up * jumpForce);
     }

     if (!grounded)
     {
         float width = transform.localScale.x;

         Vector3 newScale = transform.localScale;
         if (width > 0.5f)
         {
             newScale.x -= 0.1f;
         }
         else if (width < 0.75f)
         {
             newScale.x += 0.1f;
         }            

         transform.localScale = newScale;
     }
 }

it "pulses" because the game loop causes it to constantly execute. The player has an axis of 0.75 to start and should finish as 0.75. The cube should only contract/expand once.