Pattern adjustment, I took size 6 on the bottom of the front bodice and size 8 on the upper part. Did the same for the back.
Since my size 6 muslin was ready I just widened the upper part of the side seams (= the same adjustments I did on the pattern)
The fit is even better, it is wide enough in the bustline and fitted at the waist:
And this is my fabric, a simple lightweight cotton which I've already prewashed: