It has been stated that the symbiote amped up Spidermans's power only when it needed to. The rest of the time it did not feel the need to do so because he was already super strong. There were two times his strength was increased, that I know of. Once when fighting the Hobgoblin, Spiderman started throwing these barrels that were a few tons each with ease. Hobgoblin made a comment about it, how he seemed so much stronger. Also I think even thou it was never directly stated the costume increased his strenght when fighting Firelord. That is the only way I could see Spidey knocking him out as he did. If fighting Venom I think the symbiote would increase Spiderman's strength as much as it could. Spiderman would be much stronger and faster than Venom. Spiderman for the win.