Defense takes more resources than prosecution. The prosecution has the relative luxury of the police force doing its investigatory work - hard evidence, witness identification, etc. The defense gets discovery of a lot of that, but also has to do its own investigatory legwork regarding the facts, alibis, exculpatory evidence that wasn't turned over (or not investigated at all), friendly witnesses, etc. To make a 1:1 ratio work, you need to include the police resources used in the prosecution and allocate investigative resources to the public defenders office accordingly.

PDs tend to be good lawyers, but you're boned if you need one. Again, it's not the legal skills, it's that they lack the resources to actually dig into your case, and the sorts of people who usually need a PD are also of a socioeconomic status that doesn't have the education or resources to do the legwork.