The sectional density (weight/7000/dia squared) is a good representation of the relative length of the bullet. Since it's reasonable that varmint weight bullets are less, they will be shorter in a given calandar.

It's just one measure of killing power.

Today the bonded and particians are a much better selection than just sectional density.

A 7mm 175 grain bullet has a sectional density greater than a 300 grain
.375 H&H and it's not going to be a better killer.

There's far more than SD to decide on what bullet. Again, it's just one of many factors for choosing the best bullet.