It depends on what printer you have .
Have a look at screen snap attached and see you have such a screen
Click on the little arrow to the right of PRINT RES

Fred

snap shot 1200 ppl set

Found it.

Mical,
Here's what I see. I cannot confirm until I get the XPS driver you are using, but all my other Canon drivers will max out at 600 x 600 if you have matte type paper registered.
My IPF 8300 driver has no paper selection.
Change the paper selection to a Luster, glossy, semi gloss... and see what you get, 600 or 1200.

In the meantime, although Xmas time, I'll leave Mike a note.
Something might be special to certain drivers.

Fred

600x600 is the printer resolution which has nothing to do with the quality of the live view preview.  The live view preview uses the thumbnails so you can quickly judge the cropping and placement of the prints on the page and arrange them how you like.  To be sure you are using the highest resolution thumbnails:

(1) From the top menu, choose "View", "Thumb Quality", and choose "Best Thumbnail Quality".
(2) Then click "View", "Rebuild Thumbs/Cache" to rebuild the thumbs at the higher quality.

Step (2) is done on a per-folder basis since it rebuilds thumbs in the current folder you are viewing so if you visit a different folder of images, repeat step (2) in that folder and so on.  If in step (1) you find that it is already on best quality, you are already using the highest quality thumbnails.  In that case, you typically only see pixelization if you use tight crops in your prints.  In cases like that, if it's important to see print sharpness and you want a better idea of print quality, use the soft proof button as you've been doing: that's the best way to judge what will be printed... other than actually printing.

Mike