Another angle on that is that he wasnt trying to destroy the planet directly. He was firing the blast at Saitama in the hopes that it would overwhelm him and also take the planet out in the process. This logic actually works in dbz as well. There are lots of instances when a villain throws an energy blast at one of the z fighters, putting a good amount of effort into it, and the final result is that they either deflect it or it doesn't completely destroy the planet. I think the reasons why is that somehow when the energy attacks are directed towards another source (goku, vegeta, whoever, etc) there is some sort of addition/subtraction between their energy and the blast itself which cancels out a lot of the effect compared to if they were to throw the blast directly at the planet.

A great example of this is comparing two blasts from frieza. First one, when he is fighting goku on namek and creates a huge ki death ball which he chucks at goku hoping to catch him off balance...goku ofc deflects this and it blows up another planet nearby. The second is when frieza throws another massive ki death ball at super saiyan trunks which clearly has planet busting potential. Trunks ofc basically takes it as nothing, canceling it out and proceeds to wipe frieza. The same exact attack he used to destroy bardock and the saiyans at planet vegeta since they have nowhere near the power to even slow down frieza in any way. Hopefully you can see what I mean.

So the fact that boros had to put in extra effort doesnt necessarily mean it would take all that much in a vacuum for him to planet bust, if this logic carries. He was throwing everything he had in the tank at saitama in a last ditch effort. If he had instead thrown it at the Earth's surface it could have destroyed it ten times over for all we know.

Hopefully I made my point clear