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GCM mode already incorporates any params that could affect the outcome of the decryption. The associated authenticated data is there to allow you to rely on context for your decryption. For example, say you are encrypting some records associated with a user. You may want to include the user's database ID as the authenticated data. If a user found a way to ...

In RFC 4764, the ciphertext does not include the authentication tag created by EAX (which is treated separately); you can see this in figure 1, where they show the ciphertext (of length L) and the tag separately. In contrast, the test vectors do include the authentication tag, and that additional length make the ciphertexts look longer.

While the EAX mode permits truncating the tag to any length $\tau$ between 0 and $n$ bits, where $n$ is the block size of the underlying block cipher, this should only be taken as a statement that truncating the tag in EAX mode does not introduce any security issues beyond the obvious (an attacker only needs $2^\tau$ attempts to froge a $\tau$-bit tag by ...