Probably the most overlooked aspect of racing suit sizes is that the suit isn't supposed to fit perfectly when you're standing, but when you're in a driving position. When you take your measurements, you must make sure that the way you take them matches the way the size chart tells you. Some size charts will recommend that you take them standing up. Others will recommend taking them seated.
And almost everyone will recommend that you ask someone else to take the measurements for you, to ensure accuracy. Well, a lightweight suit doesn't just reduce the driver's overall weight, as a lightweight racing suit can increase driver comfort and help reduce the effects of fatigue on longer runs.