In order for the pie to take the whole width/height of the container you will need to:
1) Disable callouts / labels;
2) Set all margins to zero;
3) Disable slice pullout functionality.
Like this:
chart.labelsEnabled = false; chart.autoMargins = false; chart.marginTop = 0; chart.marginBottom = 0; chart.marginLeft = 0; chart.marginRight = 0; chart.pullOutRadius = 0;
Or if you are using JSON config:
AmCharts.makeChart("chartdiv", { ... labelsEnabled: false, autoMargins: false, marginTop: 0, marginBottom: 0, marginLeft: 0, marginRight: 0, pullOutRadius: 0, ... });