I used to like our coinage. Not so much any more though.
Once some thought went into what subject was worthy of a commemorative issue. Now it's all about what people will gush over and rush out to buy in its many differently minted forms.
It's good. Decent edges too, which is often a problem with these due to the weight - it only needs to be dropped once.
The large size is due to the coin being made from 2d worth of copper - 2oz.
Personally I think it's just poor quality rather than an error. If I wanted one at all I'd rather have an unflawed one.
I could be wrong but suspect this post is just a plug for your ebay listing?