With effective management, a small group of Terran units can defeat a larger amount of enemy units, especially the relatively weak Zerg breeds. In this picture you can see a very nice offensive setup. The enemy eventually had to use Queens to Broodling the Siege Tanks, and Guardians. The Science Vessel used Defensive Matrix on the two Tanks, and a Marine is up in front to spot for the Siege Tanks. SCVs are around to repair during Battle.
Marines are there to hit air targets and any unit that gets close to the Siege Tanks.
All of this requires intense micromanagement however. Try to get Siege Tanks up on high ground. They make quick work of attacking Hydralisks. Get a moving line of Siege Tanks, Marines, and possibly some Goliaths heading toward the enemy. You can get a group of SCVs to repair them as they advance.