Sectional density (SD) is a bullet's weight, in pounds, divided by the square of it's diameter, in inches. The higher the SD, the higher the weight per cross-sectional area. Shape makes no difference in the sectional density. For example, all 30 caliber (.308) bullets that weigh 180 grains have the same sectional density, .270, regardless of their shape.
A ballistic coefficient (BC) is the ratio of the sectional density of a bullet to it's form (I).
(I) is based on a ratio of drag coefficients(this bullet's to a standards bullet's). Its affected by air density, mach#, frontal cross section, shape, etc.
Ultimately, a bullet with the same shape as any standard bullet, weighing 1 lb. and 1 inch in diameter will always have a B.C. of 1.000. If the bullet is the same shape, but smaller, it will have an identical C.D, with a form factor of 1.000 and a B.C. dependant on just it's sectional density.
So to answer the question, best I can, a denser bullet with all else equal, will fly flatter(higher BC). If someone makes an equal weight solid bullet that is longer to due the material, all bets are off.