The proportionality requirement in capital punishment cases does not extend to: death sentences. parole terms. fines. prison sentences.

I believe the answer is: prison sentences.

Here is what the 8th amendment of the constitution says regarding the proportionality of capital punishment

Excessive bail shall not be required, nor excessive fines imposed, nor cruel and unusual punishments inflicted.

Prisoners that being subjected to capital punishment will definitely face a death row.

Which means that it is useless to use prison sentences as a proportionality requirement for capital punishment.