Gap between architrave and wall corner

I recently replaced a door frame to make it wider.
Now that I am fitting the architraves I find that there is an annoying gap behind the door between the architrave and the wall corner as shown in this pic:

The gap is about 3-4mm at the bottom of the gap, and flares wider at the top with the curve of the architrave.
It is going to make painting the wall and architrave very difficult, and it just annoys me.
Architraves will be white and the wall a darker sandy colour.

I was thinking of these as options to tidy it up:

• Filling the gap about half way with caulk. This would still make the left and right architraves appear the same width.
• Buy a wider architrave piece, and cut the LHS so it's just square against the corner wall. But I think this will visually unbalance the doorway, due to the high contrast between the architrave and wall colour.

If I was was going to use caulk but it could be messy and shrink given the volume needed in the gap. Would jointing compound or a plaster mix be a better option than caulk?

What would you do?

BTW - I haven't fixed the architrave into position yet.

- Karri
Attached Images

• IMG_20151207_173608472.jpg (208.1 KB)

Read and reply to the full thread at RenovateForum.com....

Gap between architrave and wall corner