18 vs 5 MB is not relevant for storing but it is relevant for downloading, since not everyone has highspeed-dsl
its not only small but its also easy to run, not only for people who already know gta2 but also for complete noobs. If you have ever been on a lanparty you know how long it takes to get a game working for everyone who wants to play it even if it is a simple procedure.
with this compilation you just need to shout "go to http://tinygta.tk/
With the original release you would have the problem that half of the people don't have 7zip; Most of the noobs will run gta2.exe instead of the manager; And if someone other that you hosts he will select a giant map and stupid gamesettings;....
Getting this working for everyone does at least take 15 minutes. And usually thats much more time than the average would invest to play a 10+ years old game. This is not for gta2 vets but its an appetizer for recruits.
with tinygta it wouldn't take longer than a minute until everyone is in the game. So it can be played while waiting for others installing another game or always where you have some minutes idle not only on lanparties but everywhere where you can find windows machines and people.
Btw: im also planing to release a fully equipped gta2 soon which will be arround 50mb
Edit: here is the first attempt: http://elypter.net84.net/gta2.tk/packs/complete_pack.7z