I have a controversial opinion
I think the Kursi or FootStool is in size proportionate to the Throne, its still smaller than it, but not insignificantly small like a Ring in a Desert type, as if it were an Atom and the Arsh the Universe, cause if Allah's Feet are on the Footstool, then since Allahs Feet are Approrpriate and Proportionate in size compared to His Other Body Parts, then it be very Strange that His Feet be upon a Footstool and that Footstool only covers 0.1% underneath God's Feet, at that point it stops servering as an Actual Footstool to put ones Feet on,the Footstool would fail to actually serve as a Footstool due to its insignificant size in Comparision to the Feet of God
Looking at the narrations, the 7 Heavens seem to be compared by Size to the Kursi, but the Kursi in some narrations is compared to the Throne in Excellence rather than Size, this appears correct to me
Abu Dharr al-Ghifārī said that the Messenger of Allāh (صلى الله عليه وسلم) said:
“The seven heavens in comparison to the Kursī is nothing but like a ring thrown in a desert, and the excellence of the ʽArsh (Throne of Allāh) over the Kursī is like the excellence of that desert over that ring.”
Abu Dharr (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: “The Kursi in relation to the Throne is like an iron ring thrown out into empty land.”
And, as one of the Salaf said, before the Throne it is like a step. (End quote from Sharh Al-`Aqidah At-Tahhawiyyah, p. 312-313)
It could also be that the Analogy doesn't intend to negate the Appropriate Proportionality of the Kursi in Relation to the Throne, usually, Thrones on Earth are much larger than Kursi anyway, so it could be emphasising the difference in size but without intending to literally make the Kursi the size of a Ring and the Throne the Size of the Desert in comparison