"not clear as" Is somewhat subjective. The difference in aperture does not seem like that much, but it is actually enough to make a meaningful difference combined with the fact that the C6 also has a larger secondary obstruction (by percentage of diameter).
Also, the tendency among testers is to compare the view at the same power. I don't know if you did this, but if you did, and you used a power of maybe 200x, this would greatly favor the larger scope because the bigger exit pupil will let the eye work better due to the added image brightness. For this comparison, using the same power in the smaller scope puts it at a disadvantage if the power is in the 1x per mm of aperture category. 200x In the 8" might still look good but in the 6", it might just be too much.
As others mentioned, it could be thermal, but seeing would have favored the smaller scope so that should be ruled out if they were compared side by side.
Collimation could be an issue as well, but no offense intended here, if you are going to compare two SCTs, it would be important to check things like that before starting.
Edited by Eddgie, 04 December 2022 - 11:12 AM.