java.lang.OutOfMemoryError

Hello, I'm ussing fop library (specifically fop-0.20.5-2004-10-20.jar). The issue is the following: When i try to render a xml file into a pdf file, and the source file (xml file)
Is longer than (more or less) 1 MB, the pdf render process blocks himself, at the instrucción driver.render((Document)domResultado.getNode()) and, after a large amount
Of time, i have an Failure of server APACHE bridge on my jsp file that called the render process. Rendering another files like html files works without any problem. What happens? Any help? Althoug i get the error above, the rendering process of the pdf file continues enternely, it's very slowy. 

Much of times, i finally get the java.lang.OutOfMemoryError and the server get down.

Why?

Thanks.
Greetings. 
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