Will reads and posts here fairly regularly, so he may well get to this post of yours pretty quick...

In the interim, I would offer the following, and Will (et al) can add there 2¢ if I have missed anything...

There is a number of reasons an author or workshop facilitator will employ larger timbers for such projects. It is usually a "first time frame" for folks...and thus...must offer a number of platforms for students to learn on and perhaps have some error in as well...

The larger sizes gives better margins from the point of view regarding engineering, as well as, to just "the feel" of working the sizes timbers (mean average) that many American Frames are often (not always) constructed from...

Size-wise, for a building you plane on constructing, the timbers could be even smaller (down in the 4x range for many of them) but the joinery system and layout would be completely different...

The question is, as your first timber frame, you may not have the skill sets to alter Will's joinery system he has for the specific frame illustrated in his book. It would be perhaps un-wise to alter sizing for a specific design without the direct input from the designer (Will) and/or seeking PE (professional engineer) approval for timber resizing and joinery alteration.

As a designer myself, I provide the frame designs engineered to be built as the plan states them. Alteration by the owner without assuming full liability themselves and/or seeking additional PE support is unwise most often....Unless one has developed the skill sets and knowledge base to do so...or...the plans give clear margins of size differential where alterations can be facilitated...Some designs and those illustrated in books come with these margins clearly stated...

Opinionated advice (mine included) here is just that "opinion" and could cover a large spectrum of experiences...good and not so much...within the craft.

Bottom line, proceeded with caution, and/or contact the author directly, or pay fees for PE resizing...I have seen dire events take place by fairly seasoned Timberwrights and General Contractors with decades of experience building "stick-frames" that attempt altering designs without knowing..." what they don't know..."

Good Luck and have fun!!!

j