Is it or is it not correct to say that the expected runtime of an algorithm is its runtime on the expected problem size? Why?
In general, it does not hold that $\DeclareMathOperator{\EE}{\mathbb{E}}\EE[f(X)] = f(\EE[X])$. For example, suppose that $X=0$ with probability $1/2$ and $X=1$ with probability $1/2$, and that $f(x) = x^2$. Then $\EE[f(X)] = 1/2$ whereas $f(\EE[X]) = 1/4$.