It's the Model Y:
-Model Y's have a larger interior space due to being taller.
-Being taller also means the windshield, side glass, and rear trunk glass is larger, letting in more sun, and therefore, more heat.
-Model 3's have a physical solid separation between the trunk and the passenger compartment making the compartment smaller and easier to cool.
That being said, tinting all the glass pieces, especially the front side windows, (and the windshield) will help a lot, but will never be as easy to cool as the Model 3.