Well in those episodes, Goku did not have the time to gather all that much energy, in the Furiza saga, he needed to take about 5 - 10 minutes to gather enough spirit energy, that's why it isn't in his hand, plus he had to hide it from Furiza anyways so it was up high when it charged cause Furiza can't read power levels without a scouter.
Vegeta wasn't all that powerful PL-10000-25000 when fighting Goku the first time on Earth. So Goku didn't need five billion ki to kill him. A small energy ball is all he needed to throw Vegeta off.