Decoupling capacitor size per datasheet
The datasheet is from 2011. The part may be even older.
Back in 2011, or when the datasheet was first made, a 0805 capacitor was typical size and good enough for decoupling.
But we can't really know the reason behind the suggestion because there is nothing that specifies why that is the suggestion.
A ceramic capacitor with smaller size has lower inductance which is better, but it may also have worse effective capacitance at the rated voltage. So if you are going to change the physical size or material of the capacitor, make sure the new capacitor is equal or better than the original.