Should size of file increase when saved on disk after extracting from packets

Question

I captured a JFIF image packet of 2817 Bytes. Out of which image was of 2497 Bytes. By using option "export selected packet bytes" in wireshark, i extracted the file to my disk. The disk size is also 2497 bytes. Should not it expand as in network transmission it was compressed by GZIP. How the size is still the same after decompressing it.

Or if it is not decompressed at all?

following is the request responce block ...

GET /vi/tN23KfbtMIA/default.jpg HTTP/1.1 
Host: i1.ytimg.com 
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.0; en-GB; rv:1.9.2.18) Gecko/20110614 Firefox/3.6.18 ( .NET CLR 3.5.30729; .NET4.0E) 
Accept: image/png,image/*;q=0.8,*/*;q=0.5 
Accept-Language: en-gb,en;q=0.5 
Accept-Encoding: gzip,deflate 
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7 
Keep-Alive: 115 
Connection: keep-alive 
Referer: http://www.youtube.com/watch?v=ERTcZV7uTFU&feature=aso
HTTP/1.1 200 OK
Content-Type: image/jpeg
Last-Modified: Thu, 01 Jan 1970 00:21:46 GMT
Date: Wed, 27 Jul 2011 10:46:19 GMT
Expires: Wed, 27 Jul 2011 16:46:19 GMT
X-Content-Type-Options: nosniff
Server: sffe
Content-Length: 2497
X-XSS-Protection: 1; mode=block
Cache-Control: public, max-age=21600
Age: 19895

========================================

Accepted Answer

No, the file size should be the same. As in this case, you talk about gzip, but the object is JPEG compressed (image) data. What you're seeing is the indication of the capability of the client to accept gzip encoded (compressed) data. It's then up to the server to decide to take the client up on its offer, or not. In this case it makes little sense, so the server chooses not to gzip the JPEG compressed data.