In preparation for Goku's arrival, Vegeta launches that crystal ball in the air and Nappa turns into an Oozaru. He is fresh and readily prepared for Goku's arrival. Once Goku arrives, Nappa will go ape shit.
Goku only has a chance to severely damage him with a Kaioken x4 Kamehameha (this is for people who agree with the 4,000 figure personally dont btw). He has a slight chance with the Genki dama as well(in which case he would win but he likely wont get the chance)... if he is interupted like with vegeta he is pretty much screwed.
Vegeta's Oozaru form was obviously extremely weakened.
a) Vegeta had been beaten up by the Kaio-Ken x3 pretty badly.
b) Vegeta get's hit with a Kamehameha wave in addition to a Kaio-Ken x4.
c) Vegeta states that throwing that ball of energy into the sky used up most of his power.
Then Vegeta transforms into an Oozaru. How did all those handicaps play out in favor of Goku? Even with the spirit bomb?