ActionScript 3.0 :: Too Much Data Being Sent To Printer

I've been building an AIR application in AS3 which includes a number of drawing tools, allowing the user to annotate a page with freehand drawings, lines, text etc. I'm almost done and everything tests well on my dev computer but testing on an old XP machine crashed every time I tried to print. Eventually I realised this was due to a mass of data being sent to the printer which that poor old XP machine just couldn't handle. All I'm doing is creating a new printJob with printAsBitmap set to true, and using addPage to add the MC I want to print.

When I look at the windows print jobs dialog I see it's sending 1.68Gb of data to the print spooler! I figured that there must be too much in the MC so I create a bitmapData of the same size and copy it to a bitmap, then add that to a blank MC and send that to the printer - same problem! The MC is 2480x3508 which equates to 300dpi on A4 paper. By my calculations that should be a bitmap of around 33Mb, so where is all the other data coming from.

Here's my PHP Code:
var myPrintJob:PrintJob = new PrintJob();
var result:Boolean = myPrintJob.start();
var options:PrintJobOptions = new PrintJobOptions();
options.printAsBitmap = true;
var thisRect:Rectangle = new Rectangle(0, 0, 2480, 3508);
myPrintJob.addPage(printTemplate, thisRect, options);

PrintTemplate is a simple MC containing a single bitmap. Both the bitmap and the printTemplate object are 2480x3508 pixels.