| First, one should realize that in general, SIP exchanges are only going to be a tiny fraction of the overall session bandwidth. A typical SIP call setup takes less than 1000 bytes, or the equivalent of one second of highly compressed
(G.729) audio. Some additional space savings can be realized by using short headers. (A realistic example for an audio call setup takes a total of about 640 bytes, of which about 69 bytes are SIP headers.)
In general, more substantive savings are possible by using either payload compression (RFC 2393) or link-layer compression, e.g., at the PPP layer. For the example above, the total size is reduced to about 520 bytes with gzip
compression.
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SIP FAQ : 
Why does SIP not have a Content-Transfer-Encoding header?