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.

The chemistry and solid state physics of the actinides is pretty interesting because of all the electrons whizzing around: consider the infrared spectrum of thoria which makes it the choice for gas light mantles or the glow in the dark behavior of uranium glass which has nothing to do with radioactivity.

Guessing: macropscopic is a typo. Most dictionaries don't have many technical words and you get use to click "add to dictionary". After a few days it's not highlighted in any of the authors or reviewers text editor and nobody notice it. If you want to get fancy, call it "click fatigue".