This is a tricky question because the diamond points are higher than the previous surface, so in one sense the diameter is larger after knurling. But there are an equal number of depressions formed, which are lower than the original surface. The average diameter is the same. The raised surface is more easily pressed down when seating the bullet or when firing it, since all you are doing is pushing down the tiny points of raised diamonds. But when this happens, the lube is squeezed up to the surface, which is exactly what you want to happen in the bore.
Many people add knurling to factory cast bullets for greater lube retention.
If you change a round object into a three equi-distant lobed shape, it remains concentric about the axis. But the amount of variance is so slight that rotating the bullet round twice automatically evens out the differences. The depth of knurling is sufficient to hold the lubricant, but not so deep that it causes permanent distortion. Place the bullet on the roller V-way, set the depth and position screws to your liking, and give the crank two complete turns. Once set, you can drop bullets onto the rollers and apply knurling at a rate of about 5-6 per minute.