The traction light is coming on to let you know it is stopping you from sliding ... the car is sensing you are or are as good as slipping, and prevents it from sliding as best as possible. If you've got to 19s, are your tyres as good as on the 17s? Also - the 17s will have a bigger side wall, soaking up some lateral energy before they break traction. The 19s don't have as much "give" in the walls.