Erlang maps can share a tuple of keys to reduce memory usage.
I found that when creating identical structures using the %Struct{...} syntax, the created structures do not share keys tuple.
But if you modify an already created default structure instead of creating a new one, the keys will be shared.

defmodule Struct do
  defstruct [:a, :b, :c, :d]
end

defmodule Test do
  def run() do
    Enum.map(0..1000, &%Struct{a: &1, b: &1, c: &1, d: &1})
    |> :erts_debug.size()
    |> IO.puts()

    Enum.map(0..1000, &%{%Struct{} | a: &1, b: &1, c: &1, d: &1})
    |> :erts_debug.size()
    |> IO.puts()
  end
end

Test.run()

Output

16016
10016

Is that %Struct{...} behavior a flaw or is it intentional?