Are there any animals that get smaller as a normal part of their life cycle?
In holometabolous insects it is quite common for the final larval stage to be heavier than the adult stage. For instance, Goliathus beetles, which are among the largest insects on earth, the adult stage often weighs about half of the final larval stage (which weigh 80-100 grams).

The same situation can be seen in the Hercules beetles, which this picture of both larvae and adult of Dynastes tityus clearly shows:

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(picture from http://harrellhouse.com/in-the-museum-eastern-hercules-beetles/)