为什么将文件读入内存是Java中内存的4倍？

我有以下代码，它读取以下文件，在每行末尾附加\r\n，并将结果放在字符串缓冲区中：

public InputStream getInputStream() throws Exception {
    StringBuffer holder = new StringBuffer();
    try{
        FileInputStream reader = new FileInputStream(inputPath);


        BufferedReader br = new BufferedReader(new InputStreamReader(reader));
        String strLine;
        //Read File Line By Line
        boolean start = true;
        while ((strLine = br.readLine()) != null)   {
            if( !start )    
                holder.append("\r\n");

            holder.append(strLine);
            start = false;
        }
        //Close the input stream
        reader.close();
    }catch (Throwable e){//this is where the heap error is caught up to 2Gb
      System.err.println("Error: " + e.getMessage());
    }


    return new StringBufferInputStream(holder.toString());
}

我尝试读取一个400Mb的文件，并将最大堆空间更改为2Gb，但它仍然给出了内存不足的堆异常。有什么想法吗？