The thickness of the troposphere varies primarily due to temperature differences, which affect air density. The correct concept here is that warmer air expands and thus raises the altitude of the troposphere, while cooler air contracts, resulting in a lower altitude. This is why the troposphere is generally thicker in the equatorial regions (where it is warmer) and thinner at the poles (where it is cooler).
However, the statement regarding water vapor weighs down the troposphere is not entirely accurate; while water vapor does affect air density, it does not significantly influence the overall height of the troposphere in the way described. Consequently, the primary reason for the variation in the thickness of the troposphere is indeed related to temperature variations.
So, the most accurate explanation regarding the variation in thickness of the troposphere is:
Cooler air contracts lowering the height, warmer air expands raising the altitude.