I don't think it's fair to say that CV was a megaton giant. I was big into video games in the NES era, and all my friends were, too. Several of them had CV1 and one rented CV2. We were all aware of the series, but CV wasn't anywhere as big as Mario, and eventually Sonic.
You have to remember that gaming was absolutely different back then. Today, gaming is recognized as a mainstream pasttime and a powerful form of media. But in the late 80's, gaming was something that young kids did. Most adults saw it as a fad; just the latest toy. Sure, they knew who Mario was, just like we know who the Teletubbies were and Dora the CrackWhora, but nobody with a wallet cared about the medium beyond wondering what their kid wanted for Christmas. So there were the big names (Mario), and a handful of smaller ones like Contra, Zelda, Ninja Gaiden, and Castlevania. Those smaller names were popular as games, but only because there was about 1% as many games available then as there are now, and those titles were the good ones. They didn't have any sort of brand power whatsoever, and they sure as hell weren't "megaton giants."