Here is the text I have in my run file:
java -Xms6G -Xmx6G -jar forge-1.8.9-11.15.1.1722-universal.jar
As you can see, I'm using Minecraft 1.8.9 and the Forge version specified in the text. I am running Pixelmon 4.2.5 on Windows 8.1. I'd like to not involve the devs here, because I'm sure it's not a problem with Pixelmon. Oh, and I'm packing 24 GB of RAM.
EDIT: Now the server's not even starting up. It gets to some [STDERR] exception, then it just closes.
EDIT2: It only starts successfully from the run.bat, and only when the limit is "1024M" and only that.
EDITZ: I was using 32-bit Java. I would kick my own rear if my legs would let me.