My understanding is that Vegeta was just dicking around. He wasn't being serious. He was trying to show as little of his strength and power as possible so he could surprise the top dog of U6, Hit. His plan didn't work because Hit was far stronger than Vegeta could detect. Magetta's mass was also never mentioned in the anime. It's strictly a Manga thing and Toyotaro doesn't understand how strong the cast is.
People do recall Ep3 of the anime, yes? What was Goku doing?

Each of those plates is easily more than a thousand tons. Add in the fact it's on King Kai's planet, so 10x that value. What Goku is benching along is easily over 60,000tons. That is a low-ball estimate. That is not counting the weighted blocks Goku and Vegeta had to lift in order to move forward on that disappearing path while training under Whis. Despite their relatively small size, those blocks far exceed even what Goku was benching above as they both struggled just to get them a few inches off the ground.
Just a few examples in their bases now it's easy to conclude that Goku and Vegeta can lift millions of tons, if not more. These are still low estimates. As we get to their training in those sumo suits, those suits were so dense and so heavy, that in the anime, when they were dropped onto the ground they began
burrowing through the ground. And continued to do so until they were out of earshot. That would indicate those suits have the mass of large islands or small continents in order to have that kind of an effect.
So the only reasons why Vegeta wouldn't be able to life Magetta are;
A) He's faking it to hide his strength
B) Magetta has the mass of a Planet.
So Magetta being said to only be 1000tons in the Manga is Toyotaro being completely oblivious to how strong Goku and co are. After all, even ignoring their lifting strength, these guys can punch hard enough to sink islands and destroy mountains. Objects that range in the hundreds of millions tons at minimum, to tens of trillions of tons. So I would say again. Unless Magetta actually possesses the mass of an entire planet, then a single serious punch from Vegeta should send him hurtling off the stage.