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It's based on a lot more than just the fastener diameter. The bolts' elasticity, whether the fastener is meant to remain elastic or be torqued to yield, the size of the head's contact area, thread pitch and actual surface area of contact of said threads (used with head size to calculate torque needed to overcome striction to achieve the REAL torque level), how clean the threads are and what lubricant is being used and a few other factors. The threads are not the weak point, if you look at a cross section of the head you'll see there is a lot of room for squish in the head... I'd be a fraction more concerned about warping the head. The torque value will be provided by the fastener's manufacturer and may vary depending on the factors above. Studs installed with clean, chased threads in the block, installed with moly grease generally are spec'd for mid 90's ft lbs. Head bolts are usually around 75ft lbs (although I think newer ARP bolts are designed for around 90 now).

The ISO spec for that fastener, just based on diameter and thread pitch and static hardness, is 115 to 120ft lbs lubed. Dry is over 150ft lbs.

I know a lot more people who have had trouble going lower than using the specs provided by the fastener manufacturer. If you're using a composite HG and the block and head are NOT finished to the point of being anywhere near smooth as glass it'll be more forgiving of a lower torque level on a stock build... the factory spec did get *most* people to at least the 100K mile mark before the HG gave up.

I believe the torque was originally decided based on the torque used in other Toyota engines using fasteners of the same or a very similar size.