This is Aces High's representation of weight and drag...
First, weight...
The more weight in an aircraft, the more momentum that aircraft has. Aces High not only models weight, but also the PLACEMENT of that weight. An aircraft with 2 500kb bombs on the ends of the wings is going to roll slower and be much tougher to control than an aircraft loaded with 1 1000kb bomb along the centerline. Same thing with fuel, it's not just how much fuel there is, but where it is.
The second thing you are noticing is a difference in drag. In many aircraft, special devices were slung off the bottom of the aircraft to allow bombs, rockets, and drop tanks to attach. Even if you drop your ordinance or drop tanks, those special adapters and fittings generally stay attached to your aircraft, meaning there is more drag along that surface. If you didn't load any bombs, rockets, or drop tanks, your plane is flying what is called a "clean" profile, and performs closer to what it was originally expected when designed.