Generated Core size blows up from seemingly small change

See main ticket #8095

With the two attached modules (Types.hs Plan.hs)

compiling to Core finishes quickly and the total Core size is reasonable:

$ time ./_build/stage1/bin/ghc -fforce-recomp -fno-code -iinput Memory.Plan -ddump-ds |wc -l
6368

real	0m2.218s
user	0m2.040s
sys	0m0.201s

However, if I change mergeGroup just slightly by uncommenting its alternate definition, i.e. by wrapping the result type in Field:

mergeGroup :: Rel '[CExposureIndex, CSameExposure] -> Field ("BAR" ::: Rel (("TRADESINGROUP" ::: Rel SameExposure_) : AffinityGroupInfo))
mergeGroup r = Field sameExps'
  where
    -- rest unchanged

then suddenly the Core size shoots up:

$ time ./_build/stage1/bin/ghc -fforce-recomp -fno-code -iinput Memory.Plan -ddump-ds |wc -l
489524

real	0m35.156s
user	0m34.467s
sys	0m2.627s

The 76x increase in Core size is coming from lots and lots (and lots and lots and lots...) of types in casts.