Light racquets are more maneuverable. Most of the new longer racquets are lighter to compensate for the extra length. Live within your means. Long strokes have large back swings and follow throughs. Medium, of course, is somewhere in between.
Compact strokes are usually less powerful than longer strokes.
The harder you swing the more power you provide. Players who swing hard usually benefit most from less powerful racquets. A player with a slower swing speed can compensate by using a more powerful racquet. More flexible racquets are also thought to be easier on the arm because they absorb more shock. Head heavy racquets are more powerful and more stable on ground strokes.