Reddit reviews A Critical History and Filmography of Toho's Godzilla Series, 2d ed.

Koichi Kawakita loved his kaiju to have a "moving mountain" feel to them, so the more massive they appeared, the more imposing he believed them to be to an audience and the easier to perform trick filmography to make them appear giant.

From an actors perspective, the suits were far more comfortable than the Showa suits. They were often molded to fit the actor exactly but were made of heavier material. This also lead to some problems; ventilation, movement range were very limited, and the suits themselves had to be a bit scaled up due to the additional padding and room needed to ensure the actor could spend longer than 3 minutes in the suit without passing out. It was more of an issue with the make of the suits than their design and the limited budget Toho had to work with.

Edited: I was in the Showa section of my "A Critical History and Filmography of Toho's Godzilla Series", so I mixed up the suit specs. Fixed it above.

David Kalat
http://www.amazon.com/Critical-History-Filmography-Godzilla-Series/dp/0786447494

and Steve Ryfle http://www.goldenageofscifi.info/pdf/Japansfavoritemonster.pdf

are probably your best bets here.

Godzilla on My Mind is a decent read, however the most serious piece of kaiju cinema criticism that I'm aware of is A Critical History and Filmography of Toho's Godzilla Series. That book is pregnant with information.

Eiji Tsuburaya: Master of Monsters: Defending the Earth with Ultraman, Godzilla, and Friends in the Golden Age of Japanese Science Fiction Film by August Ragone

A Critical History and Filmography of Toho's Godzilla Series by David Kalat

http://www.amazon.com/Critical-History-Filmography-Godzilla-Series/dp/0786447494