Its not 90% its 82% the size.
17% screen area at 16:10 is = 129.89 inches
15.4% screen area at 16:10 is = 106.59 inches
http://www.silisoftware.com/tools/screen.php
Putting my 17" MBP side (glossy) with the retina 15.4" I notice 3 things.
1) The retina is darker as the backlight is weaker (hopefully to be improved as retina displays improve and you probably wouldn't notice if you didn't see the 17" next to it).
2) The 15.4 screen is smaller, noticeably and unpleasantly smaller.
18% smaller . Did I say smaller. Its smaller.
3) Nice retina panel though. Does it come in 17"

?
ps
The 17" screen IS retina too. Retina is PPI vs distance. The 17" is retina at 26" or 66cm. I'm sat with it on my lap and the screen is farther away than that. (for the visual gag, yes that means i did stick a tape measure on my forehead

)
http://isthisretina.com/