Can I replace my 1/4" wheel bearings with slightly smaller ones?
Are you sure you're putting in the right number of balls?  Generally, when a bearing is new there is about a half-ball space left when you have a full complement of balls.  As the bearing wears this gets to be closer to a full ball's space.  It's easy to see the space and think you should put in another ball, but you don't need one.

(To hold the balls in place, of course, dab some grease into the race.  Place the balls in the grease, and install the greased cone.  This makes it fairly easy to reassemble a hub.)