As mass goes up, speed goes down

Trying to develop an Agar clone, and I've got a lot of it, but I can't quite figure out how to decrement the player's speed as its mass increases. I've tried several different ways, but nothing works. How would I make the speed go down as the mass goes up? Here's my jsFiddle. This is where I set the speed of of the players:

var playerOneMass = 36;
var player1X = (canvas.width / 2) + 50;
var player = new Player({
    x: player1X,
    y: canvas.height / 2,
    radius: playerOneMass,
    speed: {
        x: 5,
        y: 5
    },
    name: "player 1",
    dir: null
});
var playerTwoMass = 36;
var player2X = (canvas.width / 2) - 50;
var player2 = new Player({
    x: player2X,
    y: canvas.height / 2,
    radius: playerTwoMass,
    speed: {
        x: 5,
        y: 5
    },
    name: "player 2",
    dir: null
});

Solution

Let us bring some math in to help us out a little bit. When you want something to grow smaller as another grows bigger, the best option that I have found is to use an inversely proportional relationship. This will allow a smooth smaller and smaller look for you.

new_speed = scalar * start_speed / current_mass

When coming up with the scalar, I have found it best to trial and error until it looks how you want it to.

Here is an example of the equation in action utilizing Two.js.

var two    = new Two({width:320, height:180}).appendTo(document.getElementById("mytwo")),
    rect   = two.makeRectangle(100, 100, 10, 10),
    circ   = two.makeCircle(5, 100, 5),
    mass   = 10,
    rspeed = Math.PI / 10,
    mspeed = 14,
    scalar = 10;
// Make it look pretty!
rect.fill = "rgb(100,255,100)";
circ.fill = "rgb(100,100,255)";
// Looping...
two.bind('update', function(fc) {
  // Prevents from growing indefinitely
  if(mass > 150) return;
  mass += 1.5;
  rect.scale += .1;
  circ.scale += .1;
  rect.rotation += scalar * rspeed / mass;
  circ.translation.addSelf(new Two.Vector(
       scalar * mspeed / mass, 0));
}).play();

<script src="https://cdn.jsdelivr.net/npm/two.js@latest"></script>

<div id="mytwo"><div></div></div>