R. Joseph Karo in his commentary to that halacha suggests that based on how Ra'avad is quoted in Sefer HaIkarim the text may not have said "bigger and better than" in the first place:
ויש לתמוה על פה קדוש איך יקרא לאומרים שהוא גוף ובעל תמונה גדולים וטובים ממנו ואפשר שעיקר הנוסחא כמו שכתוב בספר העיקרים פ"ב ממ"א וז"ל א"א אע"פ שעיקר האמונה כן הוא המאמין היותו גוף מצד תפיסתו לשונות הפסוקים והמדרשות כפשטן אין ראוי לקרותו מין עכ"ל וגם לפי נוסחתנו זאת היתה כוונתו וכבר הרחיב הדיבור בזה בעל ספר העיקרים בפרק הנזכר
Several other commentaries state that ממנו does not refer to Rambam, but to the members of our nation. Thus, Ra'avad was not saying that the people with these beliefs were greater than Rambam. For example, R. Meir Soloveitchik in his commentary there:
נ"ב עי' ג"כ שם דהכוונה במש"כ טובים ממנו היינו מבני עמינו אבל לא קאי על הרמב"ם ז"ל
R. Moshe Shternbuch in his commentary there (Haderech L'Teshuvah p. 37) claims that there are manuscripts in which Ra'avad's statement does not contain the word ממנו at all, so again there wouldn't actually be any people greater than Rambam that believed this:
אמנם אמת שבמגדל עוז מביא גדולים וטובים ממנו אבל יש בראב"ד כת"י מדוייקים דלא גרסי כלל ממנו וא"ש בפשיטות ואם נגרוס ממנו היינו מאתנו בבית ישראל וכמ"ש ועיין אור שמח כאן
However, all of these resolutions seem to be motivated by the fact that traditional rabbinic authors simply could not conceive that there could be people greater than Rambam that believed in some form of corporeality. Yet that is not necessarily historically accurate from Ra'avad's perspective. Indeed, we know from all of Rambam's writings against corporeality that belief in corporeality was existent, if not pervasive, in Rambam's time. Combined with the fact that a quick perusal of Ra'avad's glosses shows the harsh way he spoke about Rambam in general, it is not so inconceivable that Ra'avad actually believed that (some of) those who believed in corporeality were actually greater than Rambam.
Furthermore, as I mentioned in this answer and this answer, a century or so later, R. Isaiah the Younger of Trani explicitly stated that some of the Talmudic Sages believed in corporeality. He even mentions the same reason that Ra'avad mentions – that they understood verses according to their simple literal meaning. It is entirely conceivable that Rambam's contemporaries and near contemporaries would consider Talmudic Sages to be greater than him. Thus, the answer to who are those "bigger and better than Rambam" that believed this, may be (some of) the Talmudic Sages.
