Java HotSpot Virtual Machine

Hello,
We have an application that uses Applets to perform cryptography.
Those applets are stored on the client machine.
We use Java Plug-In 1.4.2, Windows XP and IE(any version From 5.5).
To be sure the Application can crypt huge files we need to set the Java mememory to 512.
So we add a line to the deployment.properties
javaplugin.jre.params=-Xmx512m
(this line is automaticaly added when you specify the -Xmx512m parameter with the Plugin configuration tool)

But it appears that on some machines when you try to load any Applet ( on any website with an applet) the IE browser closes without any warning.
When you change the line and set the memory to 256, it works fine...
Now the question: Why? When we used those applets on other platforms they always worked, and they work on many Windows XP machines too...

Have someone had the same bug?
Is this a "well-known" bug?

Thanks.