most of h&m clothing go up to a size 44 or even 46. (on the charts it says 44 is equivalent to a US size 14 but i think euro sizes are generally a bit smaller so i'd say a 44 is a generous US 12, 36 is equivalent to a 4 or a very small 6, etc).
i do agree that clothing stores should carry more plus-size lines. my mother used to be very overweight and i'd go shopping with her and it was very frustrating for her because 99% of the clothes were ugly, unflattering and unfashionable. this may have changed in recent years but i doubt it's changed all that much. she pretty much had to go high end because they were the only ones whose clothing weren't totally ugly and were well tailored enough to be flattering on a large woman.
at the same time, if i were a designer, i wouldn't just make the same designs for smaller women into larger sizes. no offence, but leggings don't look good on most 'thin' girls, much less on someone who's overweight. the same goes for anything really tight, spandex, strapless, spaghetti straps, etc... maybe most companies don't design and make plus size clothing because they know it's more difficult and takes better tailoring to make clothes that are flattering? and since most fashionable, affordable clothes are made in sweatshops and are crap quality, that wouldn't translate well into plus sizes.