Yes, it would be a smaller attachment. The size of the file you send
would be smaller.
Examples:
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1) A file with random data. Generally the worst for compression. The
Excel file was 42.2 MB, and the WinZip file (max compression) was 23.1
MB. With 7zip, this 42.2 MB file was compressed to 16.7 MB. So you
can see that it was 55% of it's original size with WinZip and 40% with
7zip. (WinRAR would be similar to 7zip I believe)
2) A spreadsheet I actually use (ie. fairly patterned). The Excel file
was 9.52 MB -- with WinZip it was compressed (max compression) to 2.10
MB, with 7zip it was compressed to 0.78 MB. So WinZip compressed it to
22% and and 7zip compressed it to only 8% of it's original size.
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The point of all this is that your file which was 3 MB to start with
could end up as small as 0.3 MB once you've compressed it (More likely
about 1 MB if you're using WinZip).
It's this final size that ISPs are concerned about -- They do not care
about the original size of your file. Not only that, compression
places a smaller burden on the infrastructure in most cases.
For a basic outline of what compression does, see
http://en.wikipedia.org/wiki/Data_compression -- basically it works by
looking for patterns in data, with random data, there are fewer such
patterns, whereas fairly organized stuff has lots of patterns, so gets
compressed smaller.
As to which program to use, it depends on your circumstances. WinZip
(and zip programs in general) are much more widely distributed than
other compression programs (like WinRAR and 7zip), so if you have to
send it to someone else, WinZip is usually the better choice. For just
sending it to yourself, you could use any of these programs. You
probably already have WinZip on your comp, and you can download 7zip
from
www.7-zip.org . WinZip is also probably the easiest to use (since
so many have used it, and it's easy to get help if you need it).
HTH..
Scott