In November 1971 the 10th Anniversary Fantastic Four (I) #1 was celebrated by publishing many magazines with about 50% more story pages.
From FantaCo Chronicles #3: "Back in 1971, when 32 page comics were selling for 15 cents, DC comics suddenly switched over to 48 pages for a quarter, filling out the balance of their books with reprint material. It took Marvel a few months to react, but with their books cover dated November 1971, they too switched over to the new size and did DC one better by printing all new 34 page stories." (Thanks to Dan Sanders for the quote.)
Barry Pearl adds to this: "The conversion to giant-size was a costly and unprofitable thing to do. Marvel let everyone know that this was the permanent way they were going. DC signed long terms contracts to produce 25 cent comics for a year and they lost a bundle. Goodman quickly reverted back to the smaller size BUT made his comics more profitable to the retailers. This hurt DC hugely. Boy, was DC angry at Marvel."
On the other hand, the price for the extra-sized issues was raised from ¢15 to ¢25. The next issue had the normal page size again, but a prize tag of ¢20, which caused Marvel some problems because of the Wage and Price Freeze during the Nixon administration.