Is the Maine Coon the world's largest cat?

No it is not. In respect of domestic cats the Maine Coon is the largest purely domestic cat breed. It is smaller than F1 wild cat hybrids such as the F1 Savannah and F1 Safari cats.

In terms of wildcats the world's largest cat will be a liger, a tiger/lion hybrid. This is a cat that was "manufactured". Of the natural wild cats the world's largest is the Siberian tiger.

The Maine Coon is a tiny fraction the size of a tiger. It is probably the second largest domestic cat after the F1 Savannah. The world's certified (Guinness World Records) largest individual domestic cat is Magic a female F1 Savannah cat.