In a right triangle the hypotenuse is exactly twice as long as one of the legs. The smallest angle of the triangle is:

If that leg is a, the hypotenuse is $2a$ and the angle θ facing the leg has sinθ = $a/2a=1/2$, so θ = 30°. The three angles are then 30°, 60° and 90°, and the smallest is 30°. Choosing 60° picks the other acute angle, the one facing the longer leg a√3, and 45° would need the two legs to be equal, which would make the hypotenuse a√2 rather than $2a$.