Plotting an image - why does a white border appear?

Using the following code, I plot and annotate an image. But I get a white and disproportionate border around my image. Why? And how can I ask plot not to show that border?

using TestImages
using Plots

img = testimage("mandrill")
plot(img, axis=([], false))

Try playing with keyword argument size. Reducing the margins using Measures also helps to some extent.

The following code frames the mandrill more tightly:

using TestImages, Measures, Plots

img = testimage("mandrill")
plot(img, framestyle=:none, size=size(img), margins=-10mm)