Ok well that makes sense. I have the opposite problem with Demogoblin. The body I'm using has a smaller neck ball so the head was too loose until I fixed it.
Good job with the Venom, it looks good! Was the other Venom head that big that you had to melt it? Sometimes you can line the inside with a few coats of Super Glue or Clear coat nail polish and then pop the head on for a decent fit. You probably know that already.
I melted the inside of the amazing venom head sculpt so his head would fit. Damn I need a dremel tool. No more Mac Guyvering these guys anymore. Too stressful.