Why can’t I find smaller bands and straps in a size D?

Unfortunately, smaller bands are not practical (nor even strictly possible) for women with larger breasts - if the bra is supposed to actually provide any support. Bras with large cup sizes and small bands end up looking worse, as the bands crumple against the body in an attempt to support the weight of the bust.

You have to remember that a properly fitted bra depends on the band for the majority of the support to the breasts. Wider bands support heavier breasts properly, allowing you to naturally push breasts out for that more flattering silhouette. Wider bands are also less likely to ride up in the back.

Since the average set of D cup breasts weigh anywhere between 15 and 23 pounds, manufacturers naturally avoid spaghetti straps. While smaller straps are possible in a properly fitted bra, most women still tend to purchase bras with a bigger band size and smaller cup size. This fit problem puts more stress on straps. As a result, most large-busted women actually prefer wider straps.

Unfortunately the answer to your question involves a combination of the physics behind weight and support as well as the female tendency to underestimate cup size.