Lyman has said sizing down more than .003" can hurt accuracy.
The diameter of the cast bullet as it drops from the mold is effected by antimony content. More will produce a larger diameter bullet.
Molds are regulated by different alloys from each company.
Lee 10 lead 1 part tin. Lyman #2 alloy contains antimony and tin.
The bore riding part of the bullet will change in diameter with alloy. Some at castboolits may size the bore riding part also.
http://castboolits.gunloads.com