Well the problem isn't so much that there are no clothes to fit people that aren't stick thin, but to actually figure out the sizes, since they don't use one system but pretty much every size system known to man, which means that even if you find the nice dress in size 42 (I'm also of the sturdy northen European build) you don't know if it's meant in french, italian or "common european" size. They also use a system with 1-4 where 1 is the smallest (equal to S), and if you're lucky they carry sizes 3 & 4 as well. A lot of independent designers round Plaza Serrano -and elsewhere- are also big on the 'one-size-fits-all-myth'. The anarchy is in fact so big (a quilombo to speak argentine) that the goverment inforced a law a couple of years ago to make the clothes manufacturers use one coherent system, and that there should be sizes in the upper scale as well. As far as I could see, it hasn't been taken too seriously as of yet...
You just have to use your eye mesure to begin with (and probably discover, as I did, that I was quite optimistic about my size haha) and after a while you get the hang of it and know where you can find clothes you like in your size.