There are obviously many variables involved with terminal penetration, but you seem to have missed an important part of my statement: "assuming all else is equal". Equal expansion characteristics, equal impact velocity, equal bullet construction, equal impact medium, etc., the heavier bullet will penetrate further than the lighter bullet. It's a pretty simple concept, bullet mass is linearly proportional to momentum, and is also proportional to penetration.
You simply said "a heavier bullet" with all
other things being equal. If you meant that to mean that the bullets have to have the same expansion characteristics, equal construction, and equal velocity with only weight being the different, and not a general statement that heavier bullets penetrate deeper than lighter bullets of the same caliber, then that's some whacky semantics. And frankly, I think your qualifications are a bit oxymoronic. Because otherwise identical bullets of two different sectional densities aren't the same. They are different. Two bullets of the same design but with different sectional densities will likely not have the exact same expansion characteristics. But your qualification is that they must. You also make it a qualification that they are driven at the same speed. How often does that happen? For instance, with respect to the same cartridge, a 7mm bullet of a high sectional density is almost never driven at the same speed as a 7mm bullet of lower sectional density. You've created a set of qualifications that are so unrealistic that you can scarcely be wrong, but also a set of qualifications that are scarcely ever met.
