There are more energy companies (oil/gas, seismice, oil field service) concentrated in Houston than in Dallas. You are looking at only the largest oil & gas companies. You need to consider medium size companies and the whole spectrum of energy related companies.
I posted that survey for information purposes - particularly b/c the top corps were all in different cities. However, I do think the influx of $$ in N.TX will increase influence.
More spending power by consumers will spur projects, enhancements, etc. Small localities are trying to figure out how to spend their "found" money.
Would be interested in the figures for Houston vs. Dallas (or DFW Metro) if you have them.