Marcus Ericsson has revealed that he is consistently about half a second per lap slower than his team-mate Kamui Kobayashi because he is 10kg heavier than the Japanese driver.
The new regulations that came into effect this year have led to much heavier cars being used this season and several of the teams are struggling to get their race package below the allowed 691kg weight limit.
Speaking to crash.net during this week's testing session in Bahrain, the Swede admitted that his heavy physique is the major reason why is constantly being outpaced by his team-mate.
"The problem is that he's 10kg lighter than me, so for me to be able to match his times is so difficult because I have half-a-second every lap which I'm losing just because of the weight," the 23-year-old told crash.net.
"So it makes life difficult. We're working all the time to try and make the car lighter as well so it's a constant work for us, but it's for sure a bit frustrating that you're not only a rookie but you have 10kg too much over a lap.
"I don't know how much exactly but I know both cars are over the limit, and obviously he's 10kg lighter than me so my car will be quite a lot over the limit. It's losing us a lot of lap time, which is a shame."