Inlay restorations require a wider outline form than amalgam restorations to provide sufficient space for the casting material and to accommodate proper thickness for strength and durability. Unlike amalgam, which can be condensed into narrower cavities, inlays need a cleared and adequately extended margin to ensure precise seating and retention. Additionally, a wider form allows for better access for impression materials and avoids undercuts, facilitating better fit and minimizing stress concentrations. This design enhances longevity and functional performance of the inlay restoration.

Reference: Sturdevant's Art and Science of Operative Dentistry, 7th Edition.