A Symbiote bonds at the molecular level after a complete bonding. Spider-Man never completely bonded with it. Originally, the Symbiote just made the host stronger but the shape of the body (outside the head) was the human. Eddie used to pump massive amounts of metal to get his body that large.
When Venom started getting HUGE, mostly it was due to the age of the symbiote. When he fought Juggy, he fell into a vat of acid and the symbiote mutated. Notice how Carnage isnt that large and rarely gets THAT LARGE. The other symbiotes as well.
Age of the symbiote matters alot. Originally Eddie wouldnt produce anything but webbing. After a few years the symbiote was able to generate tendrils. When Carnage first appeared Venom mentioned he couldnt control his suit like Carnage could. Carnage was whipping out axes, swords and throwing axes made of symbiote matter (which broke down shortl after) Non-canonically, when The Punsher was given the suit in What If? he was able to create guns with it.
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