Without further ado, here is a comparison between getting a good fit in a bra vs getting a good fit in a shoe:
1. Sizing - Most non-bravangelized people use only the band size and the cup letter to determine which bras to try on. Many people mistakenly believe that all 34B bras are supposed to fit the same way. Likewise, most non-shoevangelized people use only the size number to determine which shoes to try on. Many people mistakenly believe that all size 7 shoes are supposed to fit the same.
2. Undersizing vs Oversizing - A lot of different studies come up with different numbers as to exactly what percentage of people are actually wearing the wrong bra size or the wrong shoe size, but all of them are in agreement that the most common bra-fitting mistake is a too-large band and a too-small cup and the most common shoe-fitting mistake is a too small shoe.
3. Aspects of Shape - As many of us know, ten different 34B bras will fit wildly differently in the shape of the cups, even if all of them have roughly the same cup volume. Commonly ignored aspects of bra shape include: level of projection, cup breadth, cup height, fullness distribution, and distance between the cups. Likewise, ten different size 7 shoes will fit wildly differently, even if they are all the same length. Commonly ignored aspects of shoe shape include: singular width, and combination width
4. What Manufacturers Make vs What Occurs in the General Population - If you have been a part of the bra fitting community for a while, you start to notice patterns in regard to which breast shapes are the most popular. More often than not, larger busted people have a narrow rooted and projected shape while smaller busted people tend towards a broad rooted and shallow shape. Matrix bra companies have a tendency to make bras all broad wired and shallow regardless of size. Similarly, the most common foot shape is NOT a singular width! Most shoes being sold today are "matrix shaped". They either come in medium singular width or if you're lucky, narrow singular width and wide singular width. The reality is, most people have combination width feet, yet most shoe manufacturers don't cater to these foot shapes! Combination width means that your forefoot width is of a different size than your heel width. For example, many people find that the the size that accomodates their forefoot width is 6W but the size that accomodates their heel width is 6N.
5. Inconsistency of size naming conventions - We all know how US bra size naming conventions are totally inconsistent. It seems that after the letter 'D', every US bra manufacturer has its own idiosyncratic size naming. Some use DD, DDD, G, some use DD, F, G, some use E, F, G. It's all a mess. Likewise, shoe manufacturers will do the same, even if their shoes come in different single widths or in combination widths. Some will use the EN, N, M, W, and EW naming convention while others will use the AAAA, AAA, AA, A, B, C, D, E, and EE naming convention. AAAA is equivalent to EN, AA is equivalent to N, B is equivalent to M, D is equivalent to W, and EE is equivalent to EW. If you need single width shoes, it might be 9AA or 9N. If you need combination width shoes, you might need 7B/AA or 7M/N.
Here is a handy chart of the above information in a condensed form:
So hopefully this helps you on your bra-fitting and shoe-fitting journeys.