The specs of that Decathlon bag say "Folded dimensions: Ø 17 x 34 cm. Volume 8 L". That is rather a lot IMO. Then again, it has a comfort rating of 0 degrees and it is dirt cheap for a down bag, so not unexpected.
For "audax bivvy use" (ie. I expect to sleep for ~ 3 hours with temperatures > 5 degrees) I'd go with a synthetic "10+ degrees" quilt.
Adding a light sleeveless down vest to a minimal bag/quilt gives a lot of flexibility: worn during sleep, still wearing it when you get up / pack your things; maybe wear it while cycling when the night is much colder than expected, using it to stay warm while fixing that puncture, etc.