The mass-energy conversion per fission is about 220 MeV, a fraction of the rest mass energy of a single nucleon. A rule of thumb is about 1 gram of 235U burnup per megawatt day, but that is not lost mass, simply conversion of 235U to fission products plus some mass to energy conversion.
Yes, key issues in waste management are: how much waste do you have (volume), and where is the waste located? With CO2 emission from coal-fired units, you have millions of tons of gaseous waste going who knows where in the biosphere, a gaseous, mobile material blown about by the four winds. With fission products, you have small quantities and you know exactly where they are (in the fuel pellets). Yes, they are radioactive for a time so you have to manage them, but a little shielding and decay time go a long way towards reducing the hazards.