Hmm, red looks bigger (Spikezilla could be leaning instead of standing upright), but he's a lot chubbier. He looks a whole lot younger than Spikezilla does: Spikezilla looks like he's fully matured, but that might be the neck rolls.
First off, there is no "American Godzilla" That abomination made in 1998 is to only ever be referred to as GINO(Godzilla I Name Only) or Zilla(Because it doesn't deserve to have the word "God" in its name.)
Secondly, the artist seems to have combined the designs of Zilla and Godzilla for Spike's design. Using Zilla's head with Godzilla's muscular frame.
Finally, the story this comic is based off of had Garble as somewhat of an Expy of Monster X from Godzilla: Final Wars, with it's second form included.