Widening is much more stable. On my 4440 it has duals and the rear is ten feet wide and the front is probably eight. It is as stable as can be. On my backhoe I keep them the same width as the bucket. It gets a little scary at times but I like knowing if my bucket fits my tractor will also. We are currently digging a pond and we make cuts across it the width of a bucket maybe 2-3 feet deep at a time. If the tires were wider than the bucket it wouldn't work.
You also have to think about implements. My 7' shredder was about useless with my 4440 because the tractor was ten foot wide and I couldn't get close enough to fences. Same thing with plows or blades they need to be wider than your implements.