Back in the old days, people talked about gears in terms of gearing "inches," which relates to how many inches the bike travels with one revolution of the crank.

To calculate, divide rear cog teeth# into front chainring teeth# and multiply by wheel diameter.

So, for a 13-tooth cog in the rear, 52 in the front, with a 27" wheel, you would get 108 gear inches (52/13 * 27). For a 26 tooth cog in the rear, with the other variables identical, you would get 54" (52/26 * 27).

When you analyze gear inches, you quickly discover that reducing or increasing the size of your chainring has a larger effect on the size of the gear than reducing or increasing the size of your rear cog by the same amount.

Does this help?

I don't know what your wrench means that certain gears would "be easier to shift into." In general, drivetrains run a bit "smoother" and there is less likelihood of dropping a chain when there is less discrepancy in size between the large chainring and the small chainring. In the rear, it doesn't make any difference.