Fallout 3 had different sizes of certain creatures, so maybe there are larger mammoths. The mammoths we've seen are still HUGE, though. I don't understand the complaint. Some people nitpick about the most ridiculous things.
While i believe mammoths to be humongous, there is one clear motive behind the complaining.
It's called Point of View.
On that pic the POV is not at the PC's usual height. Either the PC is standing on someting or mounted on a horse or other creature.
If it were, then those giants and mammoths would be seen more from underneath and appear way bigger.