Ok so here is the full code:

var NUM_CIRCLES = 30;
var BIG_RADIUS = 180;
var color = Randomizer.nextColor();
var radius = BIG_RADIUS;
var CENTER_X = getWidth() / 2;
var CENTER_Y = getHeight() - radius;

function start(){
    for (var i = 0; i < NUM_CIRCLES; i++){
        var circle = new Circle(radius);
        circle.setPosition(CENTER_X,CENTER_Y);
        circle.setColor(color);
        add(circle);
    
        radius /= i;
        CENTER_Y = getHeight() - radius;
        color = Randomizer.nextColor();
    }
}

as far as i understand you are trying to produce circles from bigger to smaller reducing radius of them by number of circles

to achive that do eachReduce = radius / numberofcircles and use it as eachReduce constant to reduce each circle by that number to achieve bigger to smaller size till zero take a look at below example

let numofcir = 10,
    radius = 80,
    reduceEachBy = radius / numofcir,
    offset = radius / 2

function setup() {
  createCanvas(640, 360);
  noLoop()
}

function draw() {
  background(0)
  stroke(255)
  noFill()

  for(let i=0;i < numofcir;i++){
    // in p5 ellipse creates circle by x and y radius
    ellipse(offset + (i * offset),150,radius,radius)
    radius -= reduceEachBy
  }
}

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