By this issue, clever readers will have seen how the claim "largest circulation of any comic magazine" might have involved a little "cookin' of the books". 1943 saw the issue go into a faster production cycle than virtually any other comic book of the time. This is, in other words, the second issue published in the month of January. With sales statistics being tallied monthly, Captain Marvel Adventures would've therefore had the potential for twice the sales of a monthly.
This is not to suggest that Captain Marvel's popularity was a "trick"—he was genuinely popular. He probably would have sold more comics, even if this were a true monthly. But it's important to point out that it wasn't a monthly during at the time that Fawcett was really pushing the "largest circulation" marketing slogan.
Comic published with a separate, miniature comic attached to the front cover. Ostensibly, Fawcett was using the popularity of Captain Marvel Adventures to introduce readers to some of their other characters. This attached comic was the thing Cap was referring to in the speech balloon on the cover.