For MPS, I had the same issue using it in 1.5.2, with MC working for like two minutes then getting laggier and laggier and finally crashing with memory errors. I added the -XX:MaxPermSize=256M to the Java VM command line and have had no issues since. MPS is half written in Scala, so the Scala stuff, like the new Forge Multipart, may just be more memory intensive in its garbage collect (where Perm memory comes into play).