scale of the axis

edit

It's because circle() in the first plot enforces aspect_ratio=1.0, as you can see by taking a look at the source code via circle??:

@options(alpha=1, fill=False, thickness=1, edgecolor='blue', facecolor='blue', 
         linestyle='solid', zorder=5, legend_label=None, legend_color=None, 
         clip=True, aspect_ratio=1.0)
def circle(center, radius, **options):
...