Actually Java applications have a maximum amount of RAM that the Java virtual machine will allow them to allocate, so just because you have RAM doesn't mean Minecraft can use it. This is why the Linux instructions say run with -Xmx1024M -Xms512M options. -Xmx specifies the maximum amount of RAM allowed, 1GB in this case and -Xms specifies the starting amount of RAM which is 512M. Unfortunately the .exe obfuscates this information from us so you can't bump it to 2048M or 4096M if you wish like you could on a Linux box.
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