You're downloading "AV definitions".
The download would normally be a "delta". The comms with Kaspersky
would be like "I have file 1234", and "oh, you need file 1235 then",
and then 1235 is downloaded. If you were to erase 1234, then I presume
they would inventory the files in the "store" they use, and you'd
end up downloading a new copy.
Actually, having had a Kaspersky subscription for a year, it's
a mess in there. They never seem to clean anything up! Whatever
mess they make, just stays there. No attempt is made to remove
unneeded files.
*******
There are a couple ways to detect malware. Signature and heuristics
(behavior). The definition files should contain signatures. They look
at the scanned files, for similarities to the megabytes of signatures
in the "store". I'm not up on all the details, but there can also be
polymorphism involved.
http://en.wikipedia.org/wiki/Antivirus_software
"Although the signature-based approach can effectively contain
virus
outbreaks, virus authors have tried to stay a step ahead of such
software by writing "oligomorphic", "polymorphic" and, more
recently, "metamorphic" viruses, which encrypt parts of
themselves or otherwise modify themselves as a method of
disguise, so as to not match virus signatures in the dictionary."
The other form of protection, is heuristics. Perhaps you get an
occasional download, which is an addition to the code base of the
scanner. And while using its real-time protection, it watches for
"evil activity".
To give an example on Kaspersky, an example I hate, Kaspersky got
in the habit of flagging *every* access to the system random number
generator. Random numbers are used as a seed for some encryption
processes. Knowing the random numbers used, might weaken encryption,
and make it possible for malware to "crack something". Well, just
about every program on the machine, was accessing the random number
generator (eight times!). And the steady flow of warning dialogs was
driving me crazy. So, that's heuristics for you. You'd be amazed "how
many yards of trip wire and frags" are loaded in the computer, when
you use an AV

It's no longer your computer - it belongs to the AV
product.
Paul