"I tried so hard to lose weight in the last couple of weeks since Barcelona (testing)," the 27-year-old told reporters on Thursday.
"Obviously I increased my weight from last year purely by training harder, putting on muscle, but I was on an extreme diet all last week and still now...I am all the time very hungry."
Formula One drivers have worked on building upper body strength over the winter to adapt to new regulations that will subject them to greater G forces in cornering as cars lap significantly faster.
The cars have a minimum weight limit, minus fuel but with driver included, which has been increased this season. The aim is to carry as little extra as possible for maximum advantage.
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Perez's French team mate Esteban Ocon said he had put on five kilos of muscle over the winter, and other drivers reported similar gains.
In past seasons, it was generally felt that every five kg of extra weight made the car 0.2 seconds a lap slower - a significant amount in a sport measured in thousandths of a second.
Perez said he had lost two or three kilos since testing ended but expected to be able to put the muscle back once the team was happier with reliability and could take weight out of the car.
(Writing by Alan Baldwin in London, editing by Ed Osmond)
