Saying that the Gaon was bigger than other gedolim isn’t because nost people said so; in the gaons time, the other gedolei yisroel said that his neshoma belonged to the rishonim, and was only here to hold back yeridas hadoros, to be a sign of what Torah used to be like. When rav aharon would said shiur, he’d say “es is shtait in aleh rishonim…rashba, rambam, gaon”
The gaon was able to make a golem at the age of 10. Rav chaim velozhiner writes that he heard from the gaon that once when sleeping, he learned over 2,000 perushim on three words of a pasuk, and with one of them, he was able to understand the shoresh of every eiver and gid in every animal in the entire world. This was on par with the arizals giluyim in his time.
This wasn’t exaggeration or praise; the gaon was unlike anyone in his time, or for several hundred years prior, and the baal hatanya was after him, so it is not unreasonable for even a chasidishe yid to say that the gaon was bigger than the baal hatanya.
As for the comparison with the chofetz chaim; I can’t comment much, and the distinction isn’t as clear to mye, only that the chofetz chaim was accepted as a gadol hador by chasidishe rebbes and litvishe gedolim alike, whereas the rashab, etc..was the rebbe of Lubavitch, and was not accepted by many other communities as the gadol hador(though he was held of by basically everyone). He was definitely outstanding among chassidishe rebbes, i have no doubt about that.