Optimus didn't technically combine with Jetfire like how robots combine in Power Rangers; Jetfire scrafice his spark and parts so that Ratchet can enhance Megatron. The source is the the comic adaptation of the movie, I picked up my copy at Target.
After Optimus was revived by the Matrix of Leadership, with him at full power still was not enough to fight off the Fallen, so with the extra parts from Jetfire he was able to destroy the sun crusher and the Fallen easily, in the process damaging Megatron and Starscream.