We hold like Rava that while the Mitzva of sacrificing the Korban Tamid begins when the shadows of evening begin to tilt or at 6 1/2 hours, the Rabbi’s ordained that it be delayed until 9 1/2 hours.
The Rambam states since Mincha was established in concomitance with the afternoon Tamid, therefore it is preferable to daven Mincha at 9 1/2 hours. This is known as Mincha Ketana.
However, the Rambam adds that since the Mitzva began at 6 1/2 hours, therefore if one davened Mincha at 6 1/2 hours he has fulfilled his obligation. This is known as Mincha Gedola.