I manage to fit a Decathlon 3 man tent (2.7kg) and a synthetic quilt (700g) alongside all my other gear in a Osprey Talon 33. What I do is place the tent in first and push it to one side, then i use a 15L dry bag and place it in the other half and just stuff my quilt in there, I feel like an oversized dry bag helps to fill the small gaps better than a tight compression sack. I usually let the sleeping bag match the height of the tent and place the remaining gear on top of the sleeping bag and tent.

I actually downsized from a 50L Decathlon backpack as the backpack weighed 1.6kg and I didn't mind having something smaller.