Vegeta said only a few select Saiyans are capable of the Power Ball, and it clearly decreases Ki.
To use Nappa as an example, suppose that Nappa's supposed inability to use the Power Ball effectively means that the Oozaru transformation simply isn't worth it should he lose the power ball's ki equivalent.
1/10 of Nappa's power would be around 780. To make his prospective Oozaru form weaker than his normal full power, let's say using the Power Ball drops him down to about 700. This means he loses 7,100 in Ki. Let's say Vegeta's Ki drain would be similar, dropping him down to < 11,000 should he perform it while in peak condition. Since Vegeta was less than optimum state after losing the beam struggle, let's put him at around 12,000 when he performed the Power Ball, making him sub-5000 after he performs it.
Then, he loses an unspecified amount of Ki/stamina upon losing his tail and the Oozaru transformation. Should he drop to about 3500, then a slightly stronger Gohan than the 2800 one who used a presumably no-amp Masenko against Nappa managing to hold his own makes sense.
Now, the only problem in that equation would be that Vegeta survived the Genki Dama even after becoming so weak, but then we have Goku saying the GD was sub-50% of its original power that he believed would be effective against Oozaru Vegeta... Who's clearly way over 40,000. Vegeta surviving it is PIS no matter how you slice it. Since logic would dictate that the GD that was to be Goku's trump card SHOULD surpass his KKx4 Kamehameha at least, yet it failed to kill a weakened Vegeta that Gohan was able to trade blows with... None of that makes sense here. So I'm going to ignore the GD altogether in this equation.