If you look at the bike in total, maximum stiffness and strength between any two loads with minimum mass is reached with a deeper than shallower triangle shape.

AFAIK if you put a load in the middle between two fulcrums, the optimum structure to support it is an equilateral triangle. With similar beam thickness, a shallower triangle with a pole sticking up or string hanging down would be weaker, because of Pythagoras' theorem. And a deeper frame would be stronger but it would also be heavier.

Think of it this way, if you have two attached points on a wall on top of each other and have to support a load one meter from the wall, you can do with a lot less strong tubes if the points are separated more vertically than less... The top tube will be in tension while the bottom one will be in compression. In fact the strength goes to infinity as the points get closer to each other.

I could draw a free body diagram.

Since the front wheel has to turn you have to connect it at the top, and there is also a load both at the seat and the bottom bracket. And the rear hub. So you connect these all together with as close to equilateral triangles and take into account the shape limitations as you can and you get the best weight for strength.

Also, small frames look really ugly with their long seat posts, girl style angled top tube construction. They are not waiting to shoot ahead like road bikes should look, more like bogged down. They are not elegant, they look lazy. Like a tractor vs a sports car. I never understood the fashion to make nonhorizontal top tubes.

Now, if you want to make a large bike with small wheels, (long head tube) I have an idea how to do it stiff too - the classical arrangement moves from triangle to a parallelogram and that's bad!