The triangles pictured are similar (SSS), that is the corresponding sides are in the same ratio. Hence the matching angles are the same.

For any right-angled triangle similar to triangle ABC, the ratio of the matching sides will be the same. It doesnâ€™t matter what the size of the right angled triangle, the ratio of the matching sides will be the same.

\begin{align}\dfrac{BC}{B^\prime C^\prime}=\dfrac{CA}{C^\prime A^\prime}=\dfrac{AB}{A^\prime B^\prime}=\dfrac{2}{1}\end{align}Notice that we can also write

\begin{align}\dfrac{BC}{CA}=\dfrac{B^\prime C^\prime}{C^\prime A^\prime}=\dfrac{4}{5}, \ \dfrac{BC}{AB}=\dfrac{B^\prime C^\prime}{A^\prime B^\prime}=\dfrac{4}{3} \ \text{and} \ \dfrac{CA}{AB}=\dfrac{C^\prime A^\prime}{A^\prime B^\prime}=\dfrac{5}{3}\end{align}Once the angles are fixed, the ratios of the sides are constant.