The brake shoes are designed like that ( not perfectly matching the contour of the brake drums) they are not the same shape as the drums for a reason. The shoes are semi-floating (not solidly attached to the backing plate) so when they make contact with the revolving brake drum the ends of the whole brake shoe compilation of parts are partially rotated in the direction of rotation.

This partial rotation forces the end of the brake shoes onto the CAM area at the upper pivot point which forces the brake shoes outward INCREASING the amount of braking force over the simple action of the hydraulic brake wheel cylinder.

This "Assist" in braking action is compounded so that the faster you are rotating the brake drum the farther the brake shoe tips are forced into the cam increasing the braking force exerted on the brake shoes, in other words the faster you are going the harder the brake shoes are pressed into the drums by the rotational forces not by the hydraulic braking system alone it's a very simple system that has been used on drum brakes since shortly after their introduction. That is why the drum brake system is actually called a hydraulic compound braking system not just a hydraulic braking system

So the brake shoes are curved slightly off exactly round to assist in this process because the curved shape enhances the forces working against the cams. The curved shape helps by drawing them in because if they were perfectly round they wouldn't achieve the same braking force from rotational forces because they would only be pulled in the same amount because they were round. When curved there is more clearance so the shoes can move more freely increasing the braking forces and it is designed that way to assist the rotational forces in pulling the semi-floating shoe assembly into the drum.

So you may think having perfect round brake shoes is your goal but if you do anything to get perfectly Fitting brake shoes you are removing a design feature that increases braking forces with no modifications. And making your shoes "More closely match the drums contour" will result in LESS braking force NOT MORE.

For our international readers with non-abs rear drum brake setups decreasing the air gap between the brake shoes and the brake drum MAY cause an unsafe condition in your car. Decreasing the air gap ALSO decreases the activation time of the rear braking system which MAY (depending on how tight you make them) cause the rear brakes to lock before the fronts. The system is designed to have the front brakes lock before the rears to prevent a floating rear portion of the vehicle due to locked brakes.

A rear brake locked before the front brakes situation makes the rear portion of the vehicle unstable due to much less frictional forces (due to liquefied burnt rubber collecting under the wheels) preventing lateral motion increasing the probability of the rear end to swing out. In simple terms if your rear brakes lock first there is a highly increased chance of the rear end getting loose and spinning your car around.

So the bottom line is to test any changes to brake systems CAREFULLY expecting violent snap changes in vehicle stability ESPECIALLY on wet road surfaces like in the rain, Test them before you trust them with your life.