Shocked at Myth #6--Bigger People have Higher Metabolisms?

Here is another myth from this link claiming to share the 20 Biggest Fitness Myths.  I find this "Myth #6" irresponsible as well.  It reads, "When people gain weight, 20-30% of that weight is muscle. Therefore, people who weigh more actually have more muscle and therefore a higher metabolic rate than those with less body weight."

Comparing a large person to a small person, probably so; but proportionately speaking, I bet not.  How would you verify something like this?  Care to discuss?