Hi Bill,
I did not find a law requiring butter to be at least a pound, but there is a federal law that prohibits retail sale of colored margarine in weights over one pound. 21 U.S.C. Sec 347(b)(2). The butter lobby has traditionally been stronger than the margarine folks, so I doubt there is a minimum butter weight.
Where did you find a federal law requiring bread to weigh one pound or more? The only thing I could find on bread weights came from the Code of Federal Regulations (21 CFR Sec. 136.3):
Definitions.
For purposes of this part, the following definitions apply:
(a) The word bread when used in the name of the food means the unit weighs one-half pound or more after cooling.
(b) The words rolls and buns when used in the name of the food mean the unit weighs less than one-half pound after cooling.
This is just a definition for purposes of interpreting the rest of the regulations, but it obviously anticipates breads with weights below one pound. I found several state laws, perhaps outdated, that required bread to be sold at certain standard weights, but those usually seemed to allow weights under a pound as well. I found a Supreme Court case from about a hundred years ago interpreting one of the state laws on weights, but it does not create any federal minimum weight standard.
Regards, Jon (who didn't enjoy practicing law, but apparently missed doing legal research!)