Hi, I believe it is because you are using Time.time in your lerp. Have a look at eric5h5's answer on this unityAnswers page: link It is very informative on this issue. The last paragraph describes why Time.time does not work, the rest has some solutions.

Hey,

Just replace Time.time with Time.deltaTime. I tested the code and it works well, alternating between shrinking and expanding the scale of a sphere.

Here's the code:

if(shrink)
{
    currentPlanet.transform.localScale = Vector3.Lerp (transform.localScale, bigScale, Time.deltaTime * 5);  

    if(currentPlanet.transform.localScale == bigScale)
    {
        shrink = false;
    }
}
else
{
    currentPlanet.transform.localScale = Vector3.Lerp (transform.localScale, smallScale, Time.deltaTime * 5);

    if(currentPlanet.transform.localScale == smallScale)
    {
        shrink = true;
    }
}

On a side note, I also sped it up to 5 instead of 0.5 so you can see the change faster. When set to 0.5 you'll get a very long period of time where the planet seems to expand/shrink VERY slowly. This is because it's waiting to reach exactly smallScale/bigScale.

Instead I would suggest waiting for a smaller number, for example:

if(shrink)
{
    currentPlanet.transform.localScale = Vector3.Lerp (transform.localScale, bigScale, Time.deltaTime * 0.5);  

    if(currentPlanet.transform.localScale.x > bigScale.x * 0.9)
    {
        shrink = false;
    }
}
else
{
    currentPlanet.transform.localScale = Vector3.Lerp (transform.localScale, smallScale, Time.deltaTime * 0.5);

    if(currentPlanet.transform.localScale.x < smallScale.x * 1.1)
    {
        shrink = true;
    }
}

Note the if conditions are waiting for a number near the target. This way you'll skip the part where the planet keeps scaling at ~0.00001 per second. Also, I only compared localScale.x, as there's no need to compare the rest since the planet is expanding the same in all axis.