that theory isn't baseless as it has been portrayed in the manga
Here you see how yatta mirror is, a flat shield, a bit bigger than Itachi, but still smaller than susanno itself..
Sasuke then throws explosive kunai to cause and explosion
You can no see the shield has grown, as the head of susanno is no longer higher in the air
specifically talking about this
You also should note the sfx showing the mirror was shrinking after the attack..
Next feat of this? pretty obvious
If you don't see it
You clearly see it extended again, covering more in the back of itachi, which is not how it originally looked, considering its flat
I can show you more.. but put one and one together.. itachi found these items.. and they have no real physical form.... when he found them I highly doubt they were the size fit for a susanno.. the sword was probably the size of a a katana and the shield the size of a normal shield.