Germany's Bild newspaper said the already-slight Nico Rosberg dropped 3 kilograms since the last race of 2013.
His Mercedes teammate Lewis Hamilton, who had a visibly muscular upper body in the last couple of seasons, has lost even more.
"Unfortunately," said the Briton who has lost 4 kilos, "getting rid of muscle is harder than getting it. It was the hardest winter of my life."
Daniel Ricciardo has also lost 4kg, with Red Bull team boss Christian Horner joking: "If it was up to Adrian Newey, our drivers would have lost about 15kg each!"
It is believed most cars are now close to if not slightly over the mandatory minimum car-plus-driver weight of 691kg.
Perhaps the lightest driver in the field is Felipe Massa, who tips the scales at just 58kg.
"I've never been as happy as I am now to be small," the little Brazilian grinned, with Bild reporting that the competitive Williams is below the 691kg limit, crucially allowing engineers to place ballast in the ideal places.
According to former F1 driver Heidfeld, however, even the lightest cars in 2014 are now way too heavy, but the weight limit is being further increased for 2015, to 701kg.
"I don't like how the cars keep getting heavier," German Heidfeld, now a Le Mans driver who last raced in F1 in 2011, told T-Online.
"In my day we were at 600kg, soon it will be 700.
"The higher weight means the cars get slower and slower -- 100kg is something like three to four seconds per lap.
"It is also against the spirit of the times, as the trend is exactly the opposite in street cars," Heidfeld added.
As for the likely pecking order in Melbourne this weekend, 36-year-old Heidfeld agrees that Mercedes looks better prepared than its rivals for 2014.
"How Ferrari will go is very speculative," he said, "but I want to emphasise that the track in Melbourne is not a good gauge anyway.
"I think we will only see a trend after the first three or four races," he added.