Adrian Sutil has revealed that Sauber are battling to get down to the minimum weight of car, which is hurting their lap times.
This season the FIA raised the minimum weight to 692kg in light of the new 1.6-litre turbocharged V6 engines and ERS.
However, teams admitted from the get-go that it would be difficult to get down to that weight.
And one team that is still struggling to do that is Sauber with reports suggesting the weight of their Ferrari engine is part of the problem.
"The weight is very hard for us to achieve at the moment," Autosport reports Sutil as having said.
"We are overweight and step-by-step we are going to improve that, [but] it's going to take some time.
"The lightweight production is expensive but for the next few races we have a plan to try to reduce [the weight] regularly."
Sutil, who is not being helped by the fact that he is one of the taller drivers in the field, has called on the FIA to increase the minimum weight.
"I cannot lose more, this is just how I am. I am a tall man so there are no other things I can do.
"Small drivers have a clear advantage. It's obvious, everyone knows it and I hope to have some adjustments in the regulations very soon.
"This is very different compared to the years before as tall drivers now have a clear disadvantage in lap time."