The book, Double Down, by Game Change authors Mark Halperin and John Heilemann, reveals insight into the Romney campaign's process for choosing a vice presidential pick, which it had dubbed "Project Goldfish."
It "was so secretive that researchers referred to the five finalists by aquatic names -- New Jersey Gov. Chris Christie (Pufferfish), former Minnesota governor Tim Pawlenty (Lakefish), Sen. Rob Portman of Ohio (Filet-O-Fish), Sen. Marco Rubio of Florida (Pescado) and Rep. Paul Ryan of Wisconsin (Fishconsin)," the authors write.
Romney, they said, ruled out Christie early, as the New Jersey governor had tendencies to show up late and his vetting file wasn't complete.
“The dossier on the Garden State governor’s background was littered with potential land mines,” they write.
But the nominee apparently was also bothered by Christie's weight -- his apparent lack of physical fitness -- even joking at his size.
"Romney marveled at Christie's girth, his difficulties in making his way down the narrow aisle of the campaign bus," the book said. "Watching a video of Christie without his suit jacket on, Romney cackled to his aides, 'Guys! Look at that!'"
Responding to the revelations, Beth Myers, who led the vice presidential search, released a statement Thursday at Christie's urging.
"Governor Christie complied fully with the Romney campaign's request for documents in a timely manner, including a complete medical report from his internist and cardiologist," she said.
Christie is widely considered a likely candidate for the Republican nomination in 2016, and is expected to easily win reelection next week for his second term as governor.