In certain cases, ghc-7.6.1 seems to have much worse memory behavior than 7.4. I have attached a simplified program that I believe demonstrates the problem.

-- compiled with ghc-7.4.2 -rtsopts -O2
./Foo  +RTS -s
200001
   3,549,666,928 bytes allocated in the heap
      55,512,376 bytes copied during GC
     335,555,096 bytes maximum residency (10 sample(s))
       3,787,576 bytes maximum slop
             453 MB total memory in use (0 MB lost due to fragmentation)

-- compiled with ghc-7.6.1 -rtsopts -O2
   2,699,298,272 bytes allocated in the heap
      55,077,544 bytes copied during GC
     327,246,968 bytes maximum residency (11 sample(s))
       3,767,408 bytes maximum slop
             858 MB total memory in use (398 MB lost due to fragmentation)

One of our applications uses 3-4GB more RAM when compiled with ghc-7.6.1 compared to 7.4, all due to memory fragmentation issues.