Great new start page . .
. . . but why such large selectors? Half the size (or less!) would still be perfectly clear and give much more content per page.
Hard to tell, my guess would be because of high DPI screens. It assures images stay sharp and the speed dials are large enough even on high resolution. Maybe they will give us the option to zoom just the speed dials in a future release. Would be a solution.
Another reason could be that speed dials show site screenshots by default. They need to be bigger so you can still read text/see icons and recognise the site.