Let me suggest you do an experiment to learn just how much "neck tension" you get with both dies. Measure the neck of two freshly sized cases with a micrometer, use a felt tip pen to mark exactly where you measured. Then seat bullets in both and measure again,same place. Pull both bullets and measure again. No matter how much difference there was empty, I doubt you will see more than maybe a half thousant difference in the pulled necks diameter, maybe no difference at all.
What you will be looking at is the fact that once brass stretches passed it's yield strength it has no more elasticity. Most necks will only have - maybe - one thou of elastic strength, any more stretching than that means nothing to bullet grip. Meaning, it's likely you are getting all the grip you can use with the Lee die and it's probably giving you straighter necks too.
The reason it's more difficult to seat in a smaller neck is you're using the bullet to expand the neck large enough to accept the bullet, that does NOT translate to more grip to resist bullet movement.