Nothing in Venoshock's movedex entry says anything about it doing more damage with Toxic than with Poison Gas or any other poisoning move. Venoshock's power against a poisoned Pokémon is 130 though, so no wonder you were OHKOing everything! The likely cause of the apparent reduced power is that when you tried it with Poison Gas, the opposing Pokémon had higher special defense, was at a higher level, or used a move to boost it's special defense or lower yours. The fact that Toxic does more damage than Poison Gas itself might have added to the total damage as well.