This could be related to a change I requested
here.
The initial browse tree (initial page load) always had small icons whilst subsequent screens had larger; I had requested that they all have the larger icons. I now have consistency across all the index selection screens but with the smaller icons. I guess the morale of the story is to be careful what you ask for :-)
It sounds like BubbleUPnP is identifying your Artist page as being an index selection page and thus not offering grid layout. What media server are you using, and is this index a default index provided by the media server or one you've defined yourself?