render image file size.

hello,
i have an odd question.
but i am wondering why a jpg at 1080x1920 is sucha a small file. not that i would like it to be larger but if i want to up load it to say a stock image web cite they are normally too small. for me a jpg runs around .3-.4 mg. so how do i get a hight dpi or quality level so i dont have to do some photoshop trickery.