Under the federal sentencing guidelines a guilty plea is given a very specific weighting. Most of the time a judge will utilize the guideline's scoring system. In that system forcing a trial would never result in a 2 year sentence growing to 20. (I assume that in most state's the process is similar.)

The real issue is the huge number of charges prosecutors can bring, charge stacking, and the rules for how aggregating factors can increase penalties. That's how a prosecutor can tip the scales so egregiously. The fix is that we need less harsh penalties and rules against charge stacking. To prevent an overcorrection we need to make sure that police departments and prosecutors enforce the [less harsh] penalties more consistently.

Penalties are so harsh today that police and prosecutors don't even care about misdemeanor convictions. If there's no potential for a felony charge the police will literally walk away. Similarly, prosecutors don't want to waste time on penny-ante misdemeanor and felony cases and will let defendants off with a minimum penalty. That creates a self-fulling prophecy that without harsh sentences criminals get off scot free. It's why California is suffering from a property crime wave--police and prosecutors are habituated to ignoring property crimes now that the penalties are less severe, which means there's no deterrence. We could have deterrence without harsher penalties if they actually did their jobs.