Here's why:

* Heating and Expansion: During summer, the ground surface absorbs more solar radiation, leading to warmer temperatures. This warmth heats the air in the troposphere, causing it to expand.

* Lower Tropopause: The tropopause, the boundary between the troposphere and the stratosphere, rises higher in the summer due to this expansion.

* Thicker Troposphere: As the tropopause moves higher, the troposphere becomes thicker.

It's important to note that this variation in thickness is more pronounced in the tropics than in the polar regions.