Diminishing returns and economies of scale
In production with more than one input, "diminishing returns" refers to what happens when we increase one input while keeping all the rest constant. "Economies of scale" is a more informal term for "increasing returns to scale" and so relates to what happens when we increase all inputs by the same proportion (while, and this is important, we keep the technology unchanged).

Perhaps it could help you to revisit your example based on the above standard definitions of these concepts.