Trunk comparison: Audi A6 Sedan vs Seat Leon

Biggest Differences
The Audi A6 has a significantly longer cargo area: with the seats folded down, it is 18.1 in (31%) longer, and with the first row upright, 12.6 in (39%) longer compared to the SEAT Leon.
The SEAT Leon, on the other hand, offers a significantly larger opening and more depth: the height of the trunk opening is 11.8 in (60%) higher, and the trunk depth (loading edge to trunk floor) is 5.1 in (108%) greater.
The loading edge of the SEAT Leon is 3.5 in (14%) higher above the ground. Regarding widths, there is a split: between the wheel arches (narrowest point), the Audi A6 is 8.3 in (26%) wider, while the width of the trunk opening is 5.9 in (16%) larger on the SEAT Leon.
Impact on Usability
For long items and larger cargo volumes on the trunk floor level, the Audi A6 is advantageous. The significantly greater interior length with folded seats and with the first row upright facilitates transporting long items (e.g., slats, long boxes, skis) and overall provides more usable cargo space.
The SEAT Leon scores with bulky, tall, and voluminous items that need to fit through the opening: the higher trunk opening and larger opening width make it easier to load tall or wide front-facing objects.
The greater depth (larger distance from loading edge to trunk floor) creates a deeper cargo compartment that offers more space for tall packages behind the loading edge but also means heavy loads must be lifted or lowered further.
The higher loading edge of the SEAT Leon (by 3.5 in) makes lifting heavy items somewhat more strenuous compared to the lower Audi A6. Conversely, the Audi A6 offers more usable width between the wheel arches; very wide items that need space between the wheel arches fit better in the A6 but might hit the narrowest point in the Leon.
However, wider items are easier to maneuver through the trunk opening of the Leon.
Conclusion: Audi A6 = better for longer and wide loads between the wheel arches as well as maximum usable trunk space. SEAT Leon = more practical for tall/voluminous loads thanks to the larger opening and depth, but with a higher loading edge and less usable width between the wheel arches.