Why does the smallest bubble size of this Highcharts jsFiddle example seem incorrect and how do I fix it?

I’m creating a bubble map and am having a problem with the smallest bubble sizes which are not behaving as I expect. After searching a bit in the documentation to understand the problem, I found this:
https://api.highcharts.com/highcharts/plotOptions.bubble.sizeBy

I don’t want to change the setting for ‘sizeBy’ from its default Area (that makes the most sense for my data) but there is helpful example alongside the API entry in JSFiddle:

https://jsfiddle.net/gh/get/library/pure/highcharts/highcharts/tree/master/samples/highcharts/plotoptions/bubble-sizeby/

Highcharts.chart(‘container’, {

chart: {
    type: 'bubble',
    plotBorderWidth: 1,
    zoomType: 'xy'
},

title: {
    text: 'Highcharts Bubbles Sizing'
},
subtitle: {
    text: 'Smallest and largest bubbles are equal, intermediate bubbles different.'
},

xAxis: {
    gridLineWidth: 1
},

yAxis: {
    startOnTick: false,
    endOnTick: false
},

series: [{
    data: [
        [1, 1, 1],
        [2, 2, 2],
        [3, 3, 3],
        [4, 4, 4],
        [5, 5, 5]
    ],
    sizeBy: 'area',
    name: 'Size by area'
}, {
    data: [
        [1, 1, 1],
        [2, 2, 2],
        [3, 3, 3],
        [4, 4, 4],
        [5, 5, 5]
    ],
    sizeBy: 'width',
    name: 'Size by width'
}]

});

Keeping in mind that I’m only nterested in showing by Area (the default), what I don’t understand about the example is why the first bubble (which has a z value of 1) is so small compared to the other bubbles. It’s area should be 0.5 times the area of the second bubble (which has a z value of 2). But it’s much much smaller (like 1/10 of the area). All of the other bubbles however seem to be proportionally correct — it’s really just that first bubble.

So my question is — why is the first bubble so small and is there a way to prevent this from happening so that all of the bubbles are proportionate according to area?

Many thanks in advance,

David