I agree. Sectional density is a good comparison between rifle caliber bullets of a smaller caliber. Comparing a .308 caliber bullet to a .284 caliber bullet, for instance. The long parallel sides of rifle bullets aid stable penetration, all other variables being equal.
But large caliber handgun and rifle bullets that have high weight, the sectional density is not as important. Momentum from their greater weight will increase penetration. A 44 caliber 300 grain XTP bullet penetrated deeper than a 500 S&W 300 grain jacketed bullet in ballistic gel on Kentucky Ballistics. The 500 S&W explosively expanded and dumped its energy but the 44 magnum 300 grain XTP did not expand and penetrated much deeper than the expanded but same weight 500 caliber bullet.
For what it is worth, too high of an impact velocity can retard penetration. A sweet spot appears to be between 1350 to 1800 feet per second. That is why BIG BORE revolvers penetrate so well I guess. They parallel that velocity range with larger caliber and heavy bullets.
A limit in weight seems to be also in play. The bullet must stabilize after leaving the barrel and some bullets can be too long or the twist rate of the barrel can affect stabilization to a great degree. The marriage of all the variables seem to be the key that we are all pursuing as we work up are loads and test our bullets.