Recently I started a project in which I needed to edit a really large XML file, but all of my editors were hitting a RAM buffer limit. To fix it, one can either change the java settings per application, or system-wide.
from a bash command line type man java
-Xmsn Specifies the initial size of the memory allocation pool. This value must be greater than 1000. To modify the meaning of n, append either the letter k for kilobytes or the letter m for megabytes. -Xmxn Specifies the maximum size of the memory allocation pool. This value must be greater than 1000. To modify the meaning of n, append either the letter k for kilobytes or the letter m for megabytes.
For the Oxygen XML editor:
On the Mac OS X platform to add or modify a startup parameter you have to right-click on the <oXygen/> XML Author application icon in Finder, in the pop-up menu select Show Package Contents, then in the Contents directory you edit the file Info.plist: in the key VMOptions you modify the parameter if it already exists in that key or you add it after the model of the existing parameters inside that key.The oxygen plist file (which is XML itself) had settings that correspond to the regular java settings mentioned above. One much change both the max buffer and initial buffer settings to open a large file.