The difference in shoe sizes genuinely confounds me, but for the clothing difference I can tell you from firsthand experience why there is a difference. I work for a large apparel manufacturer that produces millions of pieces per month across a few hundred styles. Although I'm a financial analyst, I work in our development office. When I first started, it was decided that I would make a good “fit model” for our private label men's Large tops and Medium bottoms. So, every conceivable measurement of my body was taken. After the design/development process, we receive the samples from our factories and I wear test all Large tops and Medium bottoms. Based on my feedback, the designers/developers tweak the garments. I wish I could tell you there was some scientific method behind designing the size of each garment, but it really does come down to how each garment fits the “fit model.” Considering that every company will have its own ideal “fit model,” it’s easy to see how sizing would change between companies. Side note: Not all companies use fit models. Some companies will use what they consider to be ideal sizing for each size range and create custom mannequins to check sizing. Again, how they determine “ideal” sizes is what leads to the differences in the first place. We prefer to use real people so that they can provide feedback on everything from how the seams rub against the skin to how warm the garment feels on a sunny day and even to how small or large the pouch is on a pair of underwear. FYI, it’s always too small.

However, let me add something to that. Style, purpose, target demographic, etc. all influence sizing decisions. There are significant differences in sizing for non-private label garments we produce (stuff we make for other companies). Customer A of ours, a billion-dollar wholesale team sports apparel company, prefers looser fitting clothing that won’t constrict when tucked into pants/shorts, while Customer B, a ten-million-dollar specialty apparel company, prefers garments that are slim fit, anti-microbial, moisture-wicking, etc. etc.