Boros could blow up the planet. His final attack he tried to do was meant to accomplish that, but Saitama blew it back with a punch. And even at the end of the fight we learn that Saitama still was holding back that entire fight. Honestly, and I hate to use this argument because it feels like a cop out, but I don't believe Goku and Vegeta are stronger than him simply because the entire gimmick of Saitama's character is that he is beyond OP. Nothing can give him a good fight. His whole shtick is that he's the strongest in the universe. At the very least, we can't actually measure how strong Saitama is because we've never actually seen him go all out. It's much easier for us to measure where Goku and Vegeta are since we've seen them go all out many times and lose to stronger opponents, but we just don't have that kind of measuring stick for a character who is literally just meant to be a parody of OP anime heroes. In the end I don't even think it's a fair comparison to try and make. It's like trying to decide who's the more skilled agent between James Bond and Sterling Archer.