How Big Is Big 17,18,19???

[Michael Ghia]

You can over tyre a car and give yourself less grip than with the correct size.
It's more noticable when you're on a race track and you're trying to get heat into those tyres! If they are too wide, they simply don't warm up and you slip all over the place. If they are too thin, you'll burn them out quickly.
The same applies on the road, although you rarely get to the same temperatures as on the track, you can still put too much rubber on the road for your cars weight and suspension design.
MG