Snippet from https://play.nim-lang.org

playground.nim

type Discriminator = enum
  d1, d2, d3

type Variant = object
  # 39 bytes --> 40 bytes
  f1: int32 # 4 bytes 
  case discrim: Discriminator  # 1 byte
  of d1:
    f2: float64 # 8 bytes
    f3: float64 # 8 bytes
    f4: float64 # 8 bytes
    f5: float64 # 8 bytes
  of d2:
    f6: int8 # 1 byte
    f7: int8 # 1 byte
  else:
    discard

type NonVariantD2 = object
  # 6 byte -> 8 byte
  f1: int32
  f6: int8
  f7: int8

let nv = NonVariantD2()
let v1 = Variant(discrim: d1)
let v2 = Variant(discrim: d2)
let v3 = Variant(discrim: d3)

echo sizeof(nv)
echo sizeof(v1)
echo sizeof(v2)
echo sizeof(v3)

echo sizeof(1.int32)
echo sizeof(d1)