Other than how to portray the trademark logo on your product, there is nothing in the Guide to follow a certain standard, other than the "legible enough for a reasonable person to read" standard.
WotC uses smaller fonts as they tend to cram a lot of material (mostly rules) in a certain amount of pages they ordered for a book. Third-party d20 publishers don't have to cram a lot since most of the rules are already present in the Player's Handbook, which includes the character creation rules and character advancement.
Personally, 12-pt fonts are a lot easier on the eyes but if the material is large, it will take a lot of pages to cover it, thus the price of the book will increase. It may be bad for both the publisher and their consumers.
10-pt fonts allow the writers to put in a lot of words into the product. But readers can easily be more fatigued by reading them. Trust me, I couldn't sit through more than one chapter of the Player's Handbook without dosing off. It's not that the text is boring (well, sometimes) but even with my eyeglasses, the small text seems a lot harder to read when my eyelids keep falling over the pupils.
I guess it depends. Sometimes it is like a bag of chips. The bag is big but you get a lot of air that perceive it to be full but it isn't. Giving the appearance that the book is bulky can be perceived as having a lot of contents, when it may not be, especially with the large fonts (and lots of interior artworks and illustrations).