Inner circles seem bigger than outer rings (experiment)

<!DOCTYPE html>
<meta charset="utf-8">
<style>

body {
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.axis path,
.axis line {
  fill: none;
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  shape-rendering: crispEdges;
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.stimuli {
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.stimuli.active {
  stroke: green;
}

</style>
<body>
<h1>How we perceive smaller the areas of the (colourful) rings than of their inner (white) circles</h1>
<div id="chart"></div>
<script src="https://d3js.org/d3.v3.js"></script>
<script>

var realvalues = {S: 0.075, A: 0.359, D: 0.676, F: 0.984}
var averages = {S: 0, A: 0, D: 0, F: 0}

var margin = {top: 20, right: 20, bottom: 30, left: 50},
    width = 960 - margin.left - margin.right,
    height = 400 - margin.top - margin.bottom;

var x = d3.scale.linear()
    .range([0, width])
    .domain([0,1]);
var y = d3.scale.linear()
    .range([height, 0])
    .domain([-0.5,0.7]);
    
var xAxis = d3.svg.axis()
    .scale(x)
    .orient("bottom");
var yAxis = d3.svg.axis()
    .scale(y)
    .orient("left");

var svg = d3.select("#chart").append("svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
  .append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

svg.append("g")
  .attr("class", "x axis")
  .attr("transform", "translate(0," + height + ")")
  .call(xAxis)
 .append("text")
  .attr("x", x(1))
  .attr("dx", ".71em")
  .style("text-anchor", "end")
  .text("Real values of percentage of colour");
  
svg.append("g")
  .attr("class", "y axis")
  .call(yAxis)
 .append("text")
  .attr("transform", "rotate(-90)")
  .attr("y", 6)
  .attr("dy", ".71em")
  .style("text-anchor", "end")
  .text("Errors of Estimates");
  
var line = d3.svg.line()
    .x(
    function(d) { 
    return d[0]; })
    .y(function(d) { return d[1]; });
    
zeroline = Array([x(0),y(0)],[x(1),y(0)]);
svg.append("path")
      .datum(zeroline)
      .attr("class", "zeroline")
      .attr("d", line)

stimuli = function(rate) {
  R = 30;
  irate = 1-rate;
  q = R*(1-Math.sqrt(irate))
  r = R - q/2;
  return Array(r,q);
}

d3.csv("ex1.csv", function(error, data) {

/*data.forEach(function(d) {
    d.S = +d.S
    d.A = +d.A;
  });*/
  
  //realvalues.forEach(function(realval,index) {
  points = Array();
  
    data.forEach(function(d) {
      rdata = Array();
      for (i in realvalues) {
        xx = realvalues[i] + Math.random()/50-0.01; 
        yy = +d[i]/100-realvalues[i];
        rdata.push([x(xx),y(yy)]);
        points.push([xx,yy]);
        averages[i] += yy;
      }
      svg.append("path")
      .datum(rdata)
      .attr("class", function() {if (d.group == "2") return "group-2"; else return "line"})
      .attr("d", line)
      .attr("title",d.name)
      .on("mouseover", function() {
          d3.select(this)
            .classed("active", true ) // should then accept fill from CSS
        })
      .on("mouseout",  function() {
          d3.select(this)
            .classed("active", false)
          });
    });
  
  svg.selectAll('circle')
    .data(points)
    .enter().append('circle')
    .attr("cx", function(d) {return x(d[0]) })
    .attr("cy", function(d) {return y(d[1]) })
    .attr("r", 3)
    .attr("class", "point")
    .attr("title",function(d) {return "Error of colour estimate: " + Math.round(d[1]*1000)/10 + "% (percentage points)"})
    .on("mouseover", function() {
          d3.select(this)
            .classed("active", true )
        })
      .on("mouseout",  function() {
          d3.select(this)
            .classed("active", false)
          });
    
  for(i in averages) {
      svg.append('circle')
        .attr("cx", x(realvalues[i]))
        .attr("cy", y(averages[i]/points.length*4))
        .attr("r", 10)
        .attr("class","average")
        .attr("title","Average of error of colour estimate: " + (Math.round(averages[i]/points.length*4*1000)/10) + "% (percentage points)")
        .on("mouseover", function() {
          d3.select(this)
            .classed("active", true )
        })
      .on("mouseout",  function() {
          d3.select(this)
            .classed("active", false)
          });
      st = stimuli(realvalues[i]);
      svg.append('circle')
        .attr("cx", x(realvalues[i]))
        .attr("cy", y(0.5))
        .attr("r",st[0])
        .attr("stroke-width",st[1])
        .attr("class","stimuli")
        .attr("title","Real percentage of colour: " + (Math.round(realvalues[i]*1000)/10) + "%")
        .on("mouseover", function() {
          d3.select(this)
            .classed("active", true )
        })
      .on("mouseout",  function() {
          d3.select(this)
            .classed("active", false)
          });
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});

</script>
<p>Estimates of <strong>percentage of colour in the 4 circles above</strong>  (i.e., number of black pixels vs. number of black and white pixels). Experiment on 45 university students.</p>
<p>The values on x-axis are slightly randomly moved, so the overlapted points may be visible.</p>
</p>(<em>One group was asked about the percentage of black, the other about white - no significant difference</em>)</p>

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