Sorry for the delay - Had to get the information on the TRA design window. TRA actually has specifications based on tread depth. The way TRA defines the tread depth is not always perfectly clear if your tire's tread depth is not exactly at the depth at which they define the demarcation. I'll show both the Highway tread design window and the traction tire design window.
The blue box is the design window. This window is the industry standard for tire sizing for the 35x12.50R15. Every dimension has a design window similar to this according to that dimension. As you can see, in Green, approximately the center of the window is the TRA Design size. This is essentially the nominal sizing for the dimension with the window as the allowable range of sizes.
I've plotted 7 tires on each chart - I pulled the SW and OD from the specs listed on Tirerack.com. The individual tire dimensions don't change position on the chart, only the window position changes depending on whether you choose Highway or Traction tread.
The observation from several that 'BFG is smaller diameter than others' appears to be an accurate statement (certainly for this dimension at least but keep reading). If you look at the table in the lower right (for each chart), you'll see that the difference is about 1/2 inch between the tallest tire (Grabber X3) and the two BFG models (which fall on the exact same point). You'll also note that the BFG products do fall within the window regardless whether you look at the Highway or Traction TD chart. Note that one of the 7 actually falls outside the window.
Recall that the OD and SW reflected in this chart - as well as the TRA Design size and the window - reflect a tire mounted on the design rim which is the 10" width. Mounting on a narrower rim will change the SW pretty significantly and the OD very minimally (if at all). The entire window and all the points will shift.
The range of the window can also be seen from the table. For section width the range is 11.9" to 13.4" - this is consistent for both charts. The OD range for highway tread is 33.9" to 35.1" and for traction tread it ranges from 34.1 to 35.3 - about 1/4" taller for traction tread.
Keep in mind, this is just an example using the specs for one dimension - for any other dimension, you'd have to go through this same exercise.
Back to the difference in OD - Here are the tread depths for the 7 tires I've plotted:
Note that the BFG Mud-Terrain is 18/32nds inch while most of the others are 21/32nds. That's a 3/32nd inch difference in tread depth, which is a 6/32nd difference in OD. That's just shy of 1/4" and it is solely due to the difference in tread depth. For the All Terrain - the OD difference due to tread depth is about 3/8"!
One can now begin to see that the positioning on the window is affected by the actual tread depth of the tire. If the BFG had the same tread depth as all the other tires on the chart, the difference would be less and perhaps not as noticeable when compared to some of the other tires on the chart.
🤷‍♂️
