Just like Tom, I too have increased the memory when IntelliJ asked and regretted it. This is the screen that asked for it:
However, in my case, instead of editing the .vmoptions files in the bin folder, IntelliJ created a new one in my home directory which I can only assume overrides the ones in the bin folder. I would assume this is typical of the new versions - I am using IntelliJ 2016.1 right now.
So, in order to fix this issue, go to C:\Users\<your_username>\.IntelliJIdea<version>\config (or your OS' equivalent) and delete the idea.exe.vmoptions and/or idea64.exe.vmoptions file from there. This should remove the override and have IntelliJ use the settings in the bin folder.
It might be worth checking the files in the bin folder as well, in case IntelliJ changes how it handles these settings.
Here are the default .vmoptions files in the installation's bin folder; you can't delete these as it will prevent IntelliJ from starting:
bin/idea.exe.vmoptions:
-server
-Xms128m
-Xmx512m
-XX:MaxPermSize=250m
-XX:ReservedCodeCacheSize=150m
-ea
-Dsun.io.useCanonCaches=false
-Djava.net.preferIPv4Stack=true
-Djsse.enableSNIExtension=false
-XX:+UseConcMarkSweepGC
-XX:SoftRefLRUPolicyMSPerMB=50
bin/idea64.exe.vmoptions
-Xms128m
-Xmx750m
-XX:MaxPermSize=350m
-XX:ReservedCodeCacheSize=225m
-ea
-Dsun.io.useCanonCaches=false
-Djava.net.preferIPv4Stack=true
-Djsse.enableSNIExtension=false
-XX:+UseConcMarkSweepGC
-XX:SoftRefLRUPolicyMSPerMB=50