Not as much surface speed but actual surface area will help. That 6" probably has a puny 1/4Hp motor, the 8" will have up to a 1 Hp therefore will move a lot more wheel. Heck, mine is a retired compressor motor - and it's not only big, it's a real 1Hp, not that "developed" horsepower that manufacturers are using to hide power deficiencies. It will turn something bigger then 8", but my buffs are 'safely' only 8"

Circumference is ~18.8" for the 6" and for the 8", it's 25+. This means that faster surface speed of the 8" will move 33% more surface past the spot being polished and 33% more rouge quicker to do that shining at least 33% faster. (Pi x dia will give you circumference)