Wow...this thread really erupted...much like Lavos... lol.
I don't buy Lavos not being "fully evolved" or being "the same size as his spawn". I don't think Lavos was a "child" when he came here. And by 1999 A.D. he'd probably gained all the energy he needed to erupt, destroy the earth, and begin spawning. You have to remember that while we are able to travel through time (and of course, Lavos it too), 2300 A.D. had not happened yet when Lavos erupted! Yes, he can control time, yes he has a "Pocket Dimension" or whatever, but 2300, in essence, doesn't exist yet!
I think he, being a creator of sorts for humankind/animalkind alike and the absorber of the planet's energy and the DNA of all creatures living on it, probably figured they'd reached the height of their existance by this time. He might have had the foresight to realize, that if they were aware of his presence, with the obvious technological advances evident in what we have seen of 1999 A.D., they might have been able to devise SOMETHING to go against him. I guess he didn't want to take that chance to let them get close enough to doing that, so he erupted then, destroyed everything, with any threat, however small it might have been eliminated he was free to carry out his spawning unchallenged. That is, until our band of hereos shows up.