"I know from an insider source why Ferrari is so strong," said the former Mercedes driver.
"They have found more power from their engine. The turbo has been upgraded and more recharging is happening for additional power.
"It's a small advantage at this time but Ferrari has done a good job."
Meanwhile, Rosberg has questioned Hamilton's sudden interest in football.
Hamilton said at Silverstone that he wanted to skip Saturday's press conference to watch England play in the World Cup, and he says he is keeping July 15 free in his diary so he can travel to Russia for the final.
Rosberg told Bild: "Lewis is not really a football fan. It's all a bit of a show. That's part of his image."
However, the German does think Hamilton will eventually win in 2018, notwithstanding the engine deficit.
"I keep thinking that Lewis will get the title. Mercedes is incredibly strong as a team," he said.
The season continues next weekend with the German Grand Prix.