[Blue-Truck-Nut]

If I understand your question correctly,
A nut or bolt is sized according to thread size, a 1/4" bolt will fit in a 1/4" hole. Now a bolts wrench size is measured from flat to flat on the hex, a common 1/4" bolt or nut takes a 7/16" wrench. In my understanding, fasteners are sized this way as a method of standardizing, a 1/4" bolt is a 1/4" bolt, it doesn't change size if it is a carriage bolt, a stove bolt, or an alen head. That's especially true in larger sizes, say a 3/4" bolt, which may use a 15/16" wrench on the bolt, and a 1" wrench on the nut, to facilitate the use of one wrench set in larger sizes.

I hope it helps, I'm definitely not an expert on the subject.

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