Lower initial density isn't really an advantage. If I recall correctly, alpha phase plutonium was used in early nuclear weapons cores because it was easier to shape. The brittle, harder, denser delta phase (with mechanical properties comparable to cast iron) was used in later weapons once the difficulties of machining it were overcome.

The decisions made during the Manhattan Project were not aimed at high efficiency nuclear weapons; the project needed to ensure that reliable weapons could be constructed quickly with the limited information available at the time. Even before fusion weapons were invented, atomic bombs became significantly more powerful, efficient, and lightweight due to realizing design optimizations that couldn't be tested before the war ended.