This is because Plots.jl does not have (and will never have) the same amount of information as the native backend. In this case, Plots.jl does not know about the window size that the plot will be displayed in. It will produce an image with the default size and pass that to the display, whatever that is.
EDIT:
Though the real problem here is having a static figure size to begin with, but thats a not so easy to solve problem.