DW Sizing unclear on website

I bought a watch online on the promotion however due to the size guide that's not clear or sufficient I bought the wrong size.

I bought the 32 mm "on sizing photo it looks quite big (even measured my current watch screen also 32mm) however the DW measurement include the side wall that makes it confusing as this is not clear on the website.

DW refuses to exchange it for me because it was on promotion. This is such a well established brand why is it such a problem if I want to exchange for a bigger size.

If you buy something online the sizing guide should me more clear. I would expect a better customer service if the information provided is so vague.

HP please assist me in this regard.