Wow. That Little Nemo page is GORGEOUS. Thank you for posting it. I had no idea.
And that Schomburg Subby cover definitely looks like a homage.
Big monsters ... well there was a sea serpent in one story, IIRC, that sank a Nazi dragon / sea serpent ship. I'll have to see if I can find the scan.
There's giant octopi I believe in one of the Atlas stories, I think. But not really GIANT giant.
I also want to say there's a collection of the old Atlas / Marvel monsters, but I'm not sure. Maybe I'm imagining it.