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Basically, there are two factors at play, one is the declination of the line of sight, the other is parallax.
The increased height increases the declination, or the rate at which your line of sight declines beyond the zero point. This means it takes less MOA of adjustment because the line of sight is already moving down and thus bringing the barrel up. Declination only effects vertical adjustment.
A quick calculation of the typ. 174gr .5BC bullet at 2600fps shows at 500yds with a 1.5in sight height the 'come-up' is 12MOA, while at a 4 inch height it is only 10MOA.
The second factor, parallax, is the degree to which the scope is not 'inline' with the bore, even if offset. This causes an increasing degree of error as the distance increases, and can be seen in horizontal AND vertical deviation.
Parallax can happen no matter the height, but may be increased as height increases.
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