Why compiler doesn't check the size of types with the `size` pragma?

[bather]

    type O1 {.size: 1.} = object
      a: uint64 # too huge for that size
    
    assert O1(a: 999999999'u64).sizeof == 1
    
    type O2 = object
      a: uint64
    
    assert O2(a: 999999999'u64).sizeof == 8
    
    
    
    Run

?