I don't know if you have discovered, but radio buttons rely on the LINE color and width. Try to set a field to line: none and you'll see the circles disappear! The size depends on the font size.
Make sure you check these setting in the Inspector. Also there may be values set based on the 'state' of the button, so check those, too.
Beverly is right, but be aware that in the Inspector (this is FM 14, but may apply to earlier v. as well), in the Appearance tab, at the top of the Inspector find the portion with the words in bold "Theme" and "Style" and you will see the words "Radio Button Set". Those words are actually on a drop down menu, and if you press it, you will see other choices, "Radio Button Set: Radio Button" and "Radio Button Set: Icon." You want to select the middle choice, "Radio Button Set: Radio Button." There you will see that the color of your radio button line, is set to Solid, 1 pt. and the color is light gray, which is almost totally useless. That is where you can change the color of the radio button circle to black or whatever color you like, as will as the thickness of the line.
…(this is FM 14, but may apply to earlier v. as well)…
Good reminder, Eric, about editing Radio Button components.
Object-component editing of Radio Buttons is, as you suspected, new to FM 14. Same goes for Checkbox Sets, Popup Menus, and Drop-down Lists/Cals, all of which gained component editing in 14. Definitely gives a lot more flexibility for styling these objects. Some object types, Popovers and Slide Panels for example, had component editing in 13, while others, Tab Panels for example, still are waiting for this capability.