The power of the federal government expanded in several ways and directions during World War I. It intruded massively into transportation, bending the railroads to the public need.
Of course, as far as the war itself, using a draft is a marked expansion of governmental powers. It is a direct exercise of control over individual lives.
The nature and extent of taxes collected rose sharply. (More taxes were collected, the rate of taxation changed, etc.)
New government departments were created, such as the Federal Trade Commission. This increased control over commerce in general.