Width and height are setting the size. The viewbox is setting the starting point and the units. CX and CY are the center points of the circle (in units) and R is the radius (in units).
Width: 50px
Height: 50px
VBox: 0 0 100 100
Cx: 50
Yx:
50
R: 25
Width: 100px
Height: 100px
VBox: 0 0 100 100
Cx:
50
Yx: 50
R: 25
Width: 100px
Height: 100px
VBox: 0 0 50 50
Cx: 50
Yx:
50
R: 25
Width: 100px
Height: 100px
VBox: 25 25 50 50
Cx:
50
Yx: 50
R: 25
The following are borrowed from Hunor Marton Borbely to help understand SVG shapes and functionality.