let largeSize = 200;

let currentSize = 0;

let destinationSize = smallSize;

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(5);

​

  if (dist(mouseX, mouseY, width/2, height/2) < currentSize/2){

    destinationSize = largeSize;

  } else {

    destinationSize = smallSize;

  }

  currentSize += (destinationSize - currentSize)*0.1;

  circle(width/2, height/2, currentSize);

}