Why would sbt reload gen-idea use 6GB of memory?

I'm using the command

sbt reload gen-idea

to build a largish (hundreds of thousands of lines of code) IntelliJ Scala project.
After about 30 minutes+, the program is autokilled because it runs out of memory. I checked, and it used 6GB before getting killed.
What's going on? Can I do anything to improve this?
Thanks