The Observer recorded a February 2010 circulation of 342,467, compared with 430,341 a year earlier.
It relaunched as a slimmer paper on February 21, giving it a 10% uplift in sales on the previous week, according to Guardian News & Media.
The drop in monthly circulation is exaggerated by GNM's removal of bulk copies, of which 23,571 were given out in February 2009.
GNM also claimed it had reduced the supply of copies of the European edition of The Observer from early February following a strategic review of international publishing, though it would not reveal by how much.
The figures show European edition sales were 16,882 in February, down from 29,677 in January and 35,049 in February 2009.
The Sunday Telegraph and Sunday Times also felt the impact of stopping bulk copies - the former handed out 49,853 and the latter 20,264 in February 2009.
The Sunday Telegraph's circulation dropped 10.5% year on year to 516,238 copies, while its rival's dropped 7.6% to 1,122,165.
The Independent on Sunday was hit harder, down 13.3% year on year to 155,661, despite pushing its bulks up from 39,295 to 48,783 copies.
The Mail on Sunday also increased bulks, from 69,981 to 104,068, but it was not enough to stop its circulation falling below the two million mark for the first time since March 2009, down 6.9% to 1,978,316 copies.
But it was no comfort to mid-market rival the Sunday Express, which suffered a bigger fall, down 9.5% to 575,314.
The tabloid market was the healthiest, with the biggest title News of the World falling only slightly, down 0.9% year on year to 2,993,709.
The Sunday Mirror was down 5.7% year on year to 1,155,975, but was the best month-on-month performer.
Its 2.8% increase on January was possibly due to its exclusive on Vanessa Perroncel's side of her involvement with England footballer John Terry.
The overall Sunday market was down 6.2% from 11,261,065 to 10,564,079 copies. This was a greater rate of decline than the daily market, which was down 4.4% to 10,273,835.