Which bra size is bigger 38d or 36dd?

They are sister sizes, meaning interchangeable. It depends on how you want the bra to fit. Say your band measures at 36.5 inches and your bust measures at 41.5 Inches there is a 5 inch difference between the two so an inch for every cup size (a,b,c,d,dd). If you try in the 36dd and it is a little tight around the band you could go up to a 38d to make it more comfortabl.