I write "some countries" as I'm wondering about the Spanish market. A German size 40 translates to a size 44 in Spain, IIRC, and I don't know if the Spanish sizing conventions are based on the ridiculous standards of the fashion industry or on simple statistics (i.e. a smaller clothing size for the average woman and thus a smaller plus-size market in general). However, considering that Mango is also a huge presence in Germany and sells in the Netherlands and the UK, too, they might have considered that difference in sizing for their target audience abroad.
Also, the plus-size line is kinda bland,though they do offer a few decent basics. But then we get dresses like this sad sack - for 20 Euros more than 99% of all non-sale prices on dresses in the normal fashion section. Are you fucking kidding me, Mango?