The SE who specified M16 bolts was obviously a nit.
There is no need for bolts that size - M10 will have been perfectly OK. The actual sideways 'twisting' force applied by the steel to the timber is quite small, and will be nowhere near the tensile strength of M16 bolts.
What is more important than the diameter of the bolt is the size of the washers - they have to be large enough to spread the load on the timber to prevent local crushing of the fibres.
