The default line spacing in Keynote is wickedly high. Find the spacing submenu and make it line spacing better.
So much for Apple products that are so simple and usable that you're delighted by their ease every time you use them. For some reason, this Keynote feature has troubled me for years: why is the line spacing so terribly high? And with spacing requiring intergalactic worm holes to travel between lines, why is it called "1.0" (single line) spacing?
Well, I found the option to change it. Please pop some corn, and watch me click the spacing submenu: