@Flaw Here in America we have something called vanity sizing :-)
Basically, as our population grows in size, the average for each clothing size increases too. So our women's size 0 today is larger than a size 8 was fifty years ago.
Over time, it gets harder for smaller people to find clothing that fits.
I'm tall (a giant by Japanese standards – 180cm) but I'm below average weight, so I figured the odds of getting a t-shirt from SE that fit well were pretty low to begin with, so I didn't worry too much about it.
They do "unisex" t-shirts, which are really men's shirts, so they end up very loose around the waist and look silly on me anyway.
So I just get a big one (I got a medium) and I can wear it when I do the laundry :-)