I would assume this be the regulatory requirement.
§25.969 Fuel tank expansion space.Each fuel tank must have an expansion space of not less than 2 percent of the tank capacity. It must be impossible to fill the expansion space inadvertently with the airplane in the normal ground attitude. For pressure fueling systems, compliance with this section may be shown with the means provided to comply with §25.979(b)
FAR 23 aircraft don't seem to have any explicit requirement. As
SnR notes, you will see fuel coming from vents, usually after refuelling, the cold fuel expands under the influence of the sun on the wing warming. Have seen fuel venting in high wing Cessnas in flight in steep nose down conditions following refuelling.