2.18x19" or 1.85x19" that is the question

I think mounting a full-size (250) tire would make it difficult to keep a 125 "on the pipe". Because 125's are less powerful and need lots of rpm to go anywhere fast, they require kind of a "tire spinning, ride the slide" type of riding. The factories know what they're doing by putting a narrower 100mm tire on one.........I'd stick with it.

I agree with 2-strokes 4-ever. Don't waste any power on an unecessary modification that will add a lot of unsprung weight to your bike. Unsprung weight is a killer on power and handling and will have absolutely no benefit except maybe looking kind of cool. The best kind of weight savings is usually on unsprung weight and you would be doing just the opposite. A wider wheel, tire and tube will weigh quite a bit more than the 1.85".