Yes, some shrinkage is very normal. It takes a lot out of a bulb to produce such large flowers. Just think of your bulb as a battery that needs to recharge after blooming.
To recharge your bulb, take these steps:
After all flowers have faded, clip the wilted flower from the stems 1 to 2 inches above the bulb. Do not remove the foliage.
If the plant is in a small pot, replant the bulb in a large pot, at least twice the diameter of the bulb. Use a good quality drained planters mix or peat moss.
If the bulb is already in a wide pot, replanting is not necessary.
Water regularly, using a basic plant feed every two month at a low rate.
About 6 to 7 months after flowering, move the plant to a cool location of 50F to 60F for a period of 3 months (indoors or outdoors) temperature is by far the most important.
Water very moderately to keep soil slightly moist during two months. Stop watering after these two months. The leaves may remain green, but may also gradually dry off. This is perfectly normal and not a problem.
After 3 months, move the plant back to 70F to 80F, cut off the foliage and dry the top the neck with a tissue to prevent neck-rot.
Do not water again until new leaves and flower stems appear.
After 8 to 10 weeks the Amaryllis may flower again.
Repeat these steps after every flower cycle.