OK, instead of guessing, I looked it up (what a concept, huh?)
Light beer is only slightly lower in alcohol content.
Here is a breakdown of alcohol content by beer for Michigan. Your state may be different (I know Oregon allows a higher alcohol content for beer then most other states).
Alcohol Content of Beer
Some Examples using alcohol by volume:
Budweiser ----------------- 4.66
Budweiser Light ----------- 3.43
Lone Star --------------------4.58
Lone Star Light -------------3.49
Michelob ---------------------4.95
Michelob Light ---------------4.15
Miller---------------------------4.64
Miller Light--------------------4.19
Coors---------------------------4.51
Coors Light--------------------4.00