The cell size has nothing to do with being impractical.
The SBMS0 can measure max 750A that means around 20kW for load or charge.
Then a 3000Ah x 26V = 78kWh so at max charge or discharge current posible limited by that 750A max current measurement you are at about 0.25C max charge and discharge rate.
So the reason is that with the max 20kW of charge current from max PV array you can install (you can install 30kW PV array in dual PV array setup but mac charge will still be limited to 20kW) then it will make no sense to have a larger battery than 3000Ah because it will not be useful to spend extra money on battery.