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Tires that puff out from the rim > Tires that are sucked inward from the rim.
The beauty of fat tires is you can scrape curbs and your rims will be fine. I've done in tons of times. With skinny tires you will hit the lip of your rim and curb them all up.
Not to mention that even if the sidewall is bulged out a bit it will still support the tread as opposed to the opposite that happens when the tire gets stretched.