You mean just because our population in Houston is much larger than NOLA's was? I'm not so sure about the per capita's, though.
To have the same per capita rate of crime in Houston as in New Orleans, you would have to start with approximately four times as much crime [raw numbers, not per capita] in Houston as in New Orleans.
Statistics prior to Hurricane Katrina put the population of Houston at approximately 1,953,631 compared to approximately 462,000 population for New Orleans.