There is an engineering compromise in choosing bolt sizes for tower leg applications. You want to go with large bolts for more strength in bolt shear. You want to go with small bolts so the tower leg retains most of its strength . . . every bit of material removed by the drill bit for tower leg bolt holes weakens the tower leg a little more.

I suspect that using two of the smaller size bolts would not provide enough bolt shear and using two of the larger size would weaken the joint too much / A good compromise is to use two different sizes of bolts in each leg.

I also asked why they used fine thread bolts in 25G but not in 45G or 55G.
Again, the decision was made so long ago that nobody remembers why . .

I have used coarse thread bolts in 25G many times with absolutely no problems. Coarse thread bolts have the advantage that they can be "mechanically galvanized" rather than plated.

25G tower leg bolts are well known to rust after a relatively short time exposed to the elements.

45G and 55G bolts are coarse threaded and hot dip galvanized and hold up much better in the weather.

Hot dip galvanized threads tend to fill up with zinc during the hot dip
galvanizing process and sometimes the nuts won't go on without removing some of the galvanization or galling.

Mechanically galvanized bolts hold up like hot dipped bolts but work smoothly like plated bolts, but mechanically galvanized bolts for 25G are necessarily coarse threaded, rather than fine threaded and plated - like supplied by Rohn.