In the month view, it's all down to the number of events you have on that day plus the size of the window/resolution of the screen.
If there's room, iCal will display all of an event's description, but if you shrink the window or add more events, the description will reduce in size- eventually into two columns - to give a chance of seeing all of them. Once there are more events than fit, you'll see an elipsis ( ... } at the top of the day. When this happens, you can only see ALL that day's events by going to week or day views.
You can potentially fit more in on the month view by turning-off the option to show the event time in month view in iCal Preferences.