They are not.
"A tabby is any domestic cat that has a coat featuring distinctive stripes, dots, lines or swirling patterns, usually together with a mark resembling an 'M' on its forehead. Tabbies are sometimes erroneously assumed to be a cat breed." Wikipedia
Mass: Maine Coon: 5.9 – 8.2 kg
Length: Maine Coon: 1 m
Photo Pixabay
Cats 101: Tabby Cats - 10 Interesting Facts about Tabby Cats - Animal Facts