The phrase "fatter tyres" means tires that are wider and thicker than normal.
Sentences with «fatter tyres»
Even in the lower gears the two fat tyres simply dig in and fire the car down the road. (evo.co.uk)
With its appropriately fat tyres and duck - tail spoiler, it made later models look fussy and clumsy. (evo.co.uk)
Big fat tyre upfront, relaxed handle bars, low seat height, that massive V - Twin in between, twin exhaust in the rear give it an eye popping look. (policybazaar.com)