Font size problem in Japsper report to Powerpoint file

Question

I'm using JRPptxExporter exporter = new JRPptxExporter(); to print pptx report but i'm having issue with font size its printing so small, please help me out. Here is my generate report function:

public String generateReportPPT(String name, Map params, String userName, String reportName) {

    Connection conn = null;
    InputStream stream = null;
    try {

        String placetime = "";
        javax.naming.Context ctx = new javax.naming.InitialContext();

        DataSource ds = (DataSource) ctx.lookup(
                ApplicationBean1.getJNDIDataSource());
        conn = ds.getConnection();

        ExternalContext econtext = getExternalContext();

        stream = econtext.getResourceAsStream(PREFIX + name + SUFFIX); //name

        if (stream == null) {
            throw new IllegalArgumentException("Unknown report name '" + name + "' requested");
        }

        JasperPrint jasperPrint = null;
        placetime = name + "_" + userName + "_" + System.currentTimeMillis();
        try {
            jasperPrint = JasperFillManager.fillReport(stream, params, conn);

        } catch (Exception ex) {
            throw new FacesException(ex);
        } finally {
            if (conn != null) {
                try {
                    conn.close();
                } catch (Exception ex) {
                    log("Error Closing connection ", ex);
                    throw new FacesException(ex);
                }
            }
        }

        JRPptxExporter exporter = new JRPptxExporter();
        HttpServletResponse response = (HttpServletResponse) econtext.getResponse();

        exporter.setParameter(JRExporterParameter.JASPER_PRINT, jasperPrint);
        exporter.setParameter(JRExporterParameter.OUTPUT_STREAM, response.getOutputStream());
        exporter.setParameter(JRExporterParameter.OUTPUT_FILE_NAME, name + ".pptx");

        response.setHeader("Content-Disposition", "attachment;filename=\"" + name + ".pptx" + "\"");

        response.setContentType("application/vnd.openxmlformats-officedocument.presentationml.presentation");

        exporter.exportReport();
        return new String(placetime + ".pptx");
    } catch (Exception ex) {
        throw new FacesException(ex);
    }
}