Thinking about a wider rear tire

[thumper_imp]

the ferret is right, changing size always distorts the shape of the wider tire and will mean the bike will handle different.
It is never a +.

It's also putting a level of stress on the entire tire that was never intended in the design:

The bead: Pinching a tire much more than intended (putting a tire meant for a 5.5" rim onto a 4" rim), would cause a huge amount of stress on the bead area, and the bead itself would not mate to the rim as required.

The sidewall: Tire manufacturers spend huge time and $$ designing their tires. The sidewall is meant to stand at a certain angle when mounted on the rim. Just imagine how far off that sidewall aspect would be after pinching that large tire onto a small rim.

The internal belts, bands, etc: Bending the tire farther than designed could cause internal tears and damage to the reinforcing aspects inside the tire, that would never be seen until catastrophic failure.

I could go on and on about this topic.