R我的图形大小是linux上的两倍，而不是Windows上的大小

I have a strange problem with my stacked plots, which must be outputted as .png. I have R 3.1.3 on windows xp and on linux mint ( linux is my guest OS via virtual box , with my host windows xp). I have no other problem with my linux, besides the natural sluggishness of such a set-up. I must use linux because of the better management of the locales; windows renders all my characters from various alphabets all gibberish, despite the language pack installed.I have a strange problem with my stacked plots,