It's not a great answer, but I think it's the logical one. They could have made Doctor Octopus seem like the strongest person in the universe, but if Spiderman could have knocked him out with a single punch it would have looked a little ridiculous. We would have had a film with Spiderman running away/getting beaten up the entire time, until the last fight when he landed a single punch and saved the day.
It's just a little too silly - instead, they make him seem like he can take a few blows and really can hold his own.
This also is a good cinematic technique to bolster the character of Doctor Octopus as a true danger to Peter - even when he has him pinned down and is punching him, Dr Octopus can overpower him. It heightens the sense of danger and establishes him as a worthy opponent.