Solution (source code)

= Solution

The assertion is true. The standard estimate <half-plane capacity is bounded by squared diameter> gives
$$
\operatorname{hcap}(A_n)\leq C\operatorname{diam}(A_n)^2.
$$
Consequently $\operatorname{hcap}(A_n)\to\infty$ forces $\operatorname{diam}(A_n)\to\infty$.