The size of the clitoris is not related in any way to the sexual pleasure derived during sexual activity or masturbation.
In fact, the only time the size ever has any real important is if you planned to change you gender to male and by taking testosterone hoped that the clitoris would grow large enough to work as an effective (but very small) penis.
As I assume this is not your situation, you have to remember that the clitoris will naturally change in size based on the level of sexual stimulation. When stimulated it will become filled with blood and will become considerably larger (wider and longer). It will also become more sensitive at these times because the increased blood flood helps the many nerve endings become sensitized to stimulation.
It is most likely that the clitoris is not shrinking. Instead, it may be that there is more fatty tissue around it than you used to have. This gain happen with weight gain, but it also happens with natural aging. As a result, the portion of the clitoris that is visible may be less even though the actual size of the clitoris remains the same.
Your clitoris should not continue to shrink. However, there may be subtle changes in how visible it is over time - especially when not sexually aroused. This is not a disease or illness and is completely normal.
So do not concern yourself with the apparent shrinking. It you are truly concerned you can lose weight and exercise, but as we age fat tends to deposit itself differently from when we were young so unless you are significantly overweight you may not see a difference.
I hope this helps put your mind at ease a little. Please feel free to let me know if you have any other questions about this.