This is particularly true of the rechargeable Nickel Cadmium type. The loss of capacity that you are experiencing is not unusual. Manufacturers recommend that for long life, a battery should be completely discharged before recharging. They should not be left in the charge mode longer than necessary. In real life, however, this solution is acceptable only when batteries are used with flashlights, or other non critical applications.
Nicad batteries develop a memory based on their usage history.
The optimal solution is to deep-cycle them. Thus, if you recharge them frequently before they are empty, they lose part of their capacity. This is what is meant by memory. The more yourun them down, the greater a charge they will take. Also, one enemy of batteries is heat.