You may have single wall and hers could be double wall, leaving the impression that her's is larger.

I looked at friends 3 yr old 50 gal unit. The exhaust outlet is about 3" or so and has 3" pipe. I read online that you should use exhaust pipe which is matched to exhaust outlet in diameter. If your outlet is 3.5" or so, than use 4" pipe. Makes sense. Use pipe that matches the heaters outlet under the vent hood. All generic vent hoods at home stores are stepped. They have a 3" step and a 4" step.

Most of the Rheem hot water heaters specify 3 or 4 inch vent size, so you can use either.

My point was that a 3" single wall galv. vent pipe is the same dia. as a 3" double wall, interior dia., however, the double wall has a larger overall dia.

Besides which, double wall can be code required depending upon the installation.

There is also, no loss of efficiency in using a 4" pipe, single or double wall.